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Hot Rods 305 V6 to SBC 350 swap?

Discussion in 'The Hokey Ass Message Board' started by Fourdy, Aug 12, 2019.

  1. Fourdy
    Joined: Dec 9, 2001
    Posts: 454

    Fourdy
    Member

    I have a 1965 GMC 2500 with V6 and 4 spd that I would like to swap a 350 SBC, 400 trans and Gear Vendors O/D into.

    Have any of you ever made this swap. Is it a bolt in, such as motor mounts? I am pretty sure I will have to make a rear transmission crossmember?

    I have been told that these rigs also came with a 283 at the time so I should be able to find motor mount brackets that will bolt into the engine bay.

    I appreciate your help and info.

    Fourdy
     
  2. Casey Riley
    Joined: Jun 27, 2018
    Posts: 519

    Casey Riley
    Member
    from Minnesota

    Never seen a 305 V6...
     
  3. Vimtage Iron
    Joined: Feb 28, 2010
    Posts: 552

    Vimtage Iron
    Member

    I don't believe GMC came with a 283, they had the V-6's and a straight six, I'm guessing Chevy came with the 283 but those trucks were different animals under the hoods, I don't believe its a bolt in swap, I'm not familar with the Chevys of that era, we had a GMC with that V-6 in that time frame.
     
  4. We've seen SBC's installed in everything under the sun. I don't think this one would be much of a challenge.
    Also, I don't believe the frames in those years are much different, if at all.
     
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  5. 56sedandelivery
    Joined: Nov 21, 2006
    Posts: 6,695

    56sedandelivery
    Member Emeritus

    The Chevrolet and GMC frames should basically be the same, therefore, motor/engine mounts will bolt/weld in; it's really just a matter of finding them these days. I'd start with Hemming's if you don't have a large truck wrecking yard nearby. The trans/Gear Vendors OD will take more work; the trans cross-member will have to be moved back, a different slip yoke used, and the driveshaft/driveshafts shortened; if your truck has a 2 piece driveshaft, then the front "half" will need to be shortened. At that point, it may be best to just go with a single driveshaft due to the added length of the Gear Vendors OD. The GMC V-6 is a totally different V-6 than the modern day V-6 from G.M. It is a TORQUE motor, and was even used in LARGE trucks by combining two of them together in a single casting, to make a V-12. I'm sure someone will jump in with a few photos of those motors. They were terrible when it came to gas mileage. And, there are no aftermarket performance parts for them. I think most would consider them a mooring device for ships and boats to keep them in place. I am Butch/56sedandselivery.
     
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  6. TrailerTrashToo
    Joined: Jun 20, 2018
    Posts: 1,044

    TrailerTrashToo
    ALLIANCE MEMBER

    Back in the mid 1970's, I had a 305 V6 in a one ton GMC. It had a wonderful exhaust note, but, was quite thirsty.
     
  7. Why the TH400 and aftermarket o/d? Why not use a 200 4 R or a 700R4 o/d, if I might ask.
     
  8. Mr48chev
    Joined: Dec 28, 2007
    Posts: 29,769

    Mr48chev
    ALLIANCE MEMBER

    The basic small block to replace the heavy gas guzzling 305 has been done untold times since the mid 60's. Around here the prime reason for swapping them was the big cost of rebuilding the 305 six.Farmers could go get the V8 and most of the needed pieces from a wrecking yard for a lot less than having the six redone.
    I'm thinking the main things you need for the proposed swap is a set of correct front motor supports (63/72 Small block work. and a new crossmember for the back of the trans and rework the driveshaft.
     
  9. Casey Riley
    Joined: Jun 27, 2018
    Posts: 519

    Casey Riley
    Member
    from Minnesota

    I never knew about a V6 that big.
     
  10. Riley, you ARE a youngster. Well, compared to some of us. A cousin in Arkansas had a 12 used in logging. Mean pulling machine.

    Ben
     
    studebaker46 likes this.
  11. scrubby2009
    Joined: Jan 9, 2011
    Posts: 198

    scrubby2009
    Member

    Had a '65 GMC with the 351v6. Factory 3/4t 4wd. Towing machine, amazing off idle torque and with 12" glasspacks on full dual exhaust, it sounded sweet. No PS or PB though, got old quick as a daily driver.
     
  12. Ive actually done the swap. It can be a bolt in. you will only need to drill several 3/8 bolt holes. Any front frame mounts from a 63 thru 66 six or V8 chevy or GMC pickup will bolt into your GMC frame. Beginning in 63 the chevy inline six and V8 engines where available in GMC Trucks.. You can get the rear crossmember from a automatic 63 thru 66 truck and it will be a bolt in. You will need to remove the front passenger side V6 engine mount bracket from the frame because it interfer's with the fuel pump on a V8 engine.
     
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  13. '51 Norm
    Joined: Dec 6, 2010
    Posts: 745

    '51 Norm
    Member
    from colorado

    I did a similar swap many years ago. We kept the manual transmission in the truck and that complicated the swap; keeping a transmission that is already bolted to the v8 will make for an easier swap.
    That said nothing is a bolt in. Yes you can get the engine onto the motor mounts but then dealing with all of the other stuff is a pain. You will need all of the bracketry for the alternator, etc. As I recall the radiator hoses, throttle linkage and exhaust all need to be changed.
    When we started the project we were told that "it'll just bolt right in". That became something of a joke as the project progressed.
    It can be done and has been many times but don't expect it to go without finding that you need to make or modify a bunch of little things. That is the nature of hot rodding.
     
    Deuces likes this.
  14. They made the V6 in 305 ,351,401 and 478 cubes. and a 702 V12
     
    Truckdoctor Andy likes this.
  15. I suppose for most folks it might not be a bolt in? I can go right out back and gather the parts from my hoard to do the swap a dozen times. There is even a 66 GMC that has the factory 250 six engine and a powerglide automatic in my hoard. Theres a chevy that had a factory 327 and and turbo 400 trans.
     
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  16. southcross2631
    Joined: Jan 20, 2013
    Posts: 4,414

    southcross2631
    Member

    When my dad worked as a mechanic for the highway dept in my home county in northern Mich. There whole fleet was V-6 GMC . From dump trucks to pickups. He said there snow plows would get 2 to3 mpg at 28 mph. plowing snow. They converted their pickups to inline 6's because of the poor gas mileage.
     
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  17. The 305/351 GMC V6 engines is a boat anchor, Gas Hungary thirsty pigs. Yes they was torque monsters and that is about it. And for the big over the road trucks there was a 702 V12, 2 351's casted together.

    Years ago a farmer was going around the country buying every V12 he could find so he could have a fleet of them in his trucks running on propane That's how I learned about these beasts. Went to S Phx and got a couple to resale, Could only carry one at a time in the one ton, Squatted it down on the over loads they was so heavy.

    The OP wants to do a v8 and t/400 with a gear vendors O/D interesting choice, As stated above why not a 700r or such? Unless he has the parts just laying around waiting to do something.

    Yes you will have to redo a transmount for what ever auto you are putting in , GMC didn't have a SBC in there trucks Until the 67 model year.

    Did a swap years ago myself and if I recall I did use the Chevy mounts for the front. Changed the bell housing for the 4 speed to the chevy nothing was just bolt in..
     
  18. Deuces
    Joined: Nov 3, 2009
    Posts: 20,903

    Deuces
    Member
    from Michigan

    Did someone mention a gmc V-12????.... Cylinder-block-and-related-parts-GMC-702.jpg GMC-Twin-Six-GM-photo1.jpg Twin-Six-left-side.jpg GMC-v12ad-400x530.jpg GMC-Twin-Six-truck-engine-bw.jpg a4fb11dc63d789e29af42ac10782d948.jpg
     
  19. Yes GMC did have both inline sixes 230 -250 and 283 and 327 engines with automatics beginning in 63. I happen to have a couple in the hoard. The GMC frames do not have a Vin stamped on them. and most after 63 are rear leaf spring. However the 63 trru 66 GMC and chevys have the same front suspension & crossmember . a stick V8 chevy bellhousing will bolt to the GMC stick crossmember frame mount. the chevy rear auto trans rear crossmember is identical to the GMC auto trans rear crossmember. I at present have at least 75 of the 60 thru 66 Chevy and GMC trucks at this very moment. With the correct parts its a bolt in.
     
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  20. arkiehotrods
    Joined: Mar 9, 2006
    Posts: 6,510

    arkiehotrods
    ALLIANCE MEMBER

  21. jazz1
    Joined: Apr 30, 2011
    Posts: 1,498

    jazz1
    Member

    I had a 305 V6 in a early 70’s GMC straight truck. Pulled great. Lost air pressure and coasted near 5 miles to get stopped travelling through rolling grade in NW Ontario. Air brakes but no maxis, was interesting. The coat hanger holding compressor tension had broke and no air warning buzzer either.
    Tapped the brake pedal only to see air drop from 70 to 0:eek:
     
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  22. Fourdy
    Joined: Dec 9, 2001
    Posts: 454

    Fourdy
    Member

    Oh nooooo, just realized that if I take the 4speed out I have lost my E-brake. (brake band on the trans) What to do????

    Bowtie just advised me that their 700-r4 level 2 would be more than adequate to handle the load and not require the gear/vendor o/d
     
  23. just get a 55 thru 59 V8 bellhousing or a 63 thru 69 six or V8 bellhousing it will bolt directly to your current V6 trans and rear crossmember. Most trucks used a cable operated emergency brake on the rear end. The best thing is to get the front suspension and rear end from a 73 and newer truck and install it on your frame. you get disk brakes and 5 lug hubs. front and rear ends 001.JPG
     
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  24. Mr48chev
    Joined: Dec 28, 2007
    Posts: 29,769

    Mr48chev
    ALLIANCE MEMBER

    Just how big of a truck are you dealing with? Up through 1 ton all had a foot brake park brake as far as I know and you would have to have a 1-1/2 ton or bigger to have a hand brake next to the shifter with the brake on the back of the trans.
    We need a lot more info than you have given us and Old Wolf pretty well detailed it out as far as 1/2 or 3/4 go.
     
    Deuces likes this.
  25. 64 and older with the SM420 has a hand brake on the trans. 65 and newer used a under the dash lever to actuate the cable parking brakes on the rear end.
     
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  26. Mico Lock and Park ..What more do you need?
    Glad someone told you you don't need a Gear Vendors.;)
     
  27. Fourdy
    Joined: Dec 9, 2001
    Posts: 454

    Fourdy
    Member

    This is the rig I am talking about.
     

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  28. Fourdy
    Joined: Dec 9, 2001
    Posts: 454

    Fourdy
    Member

    mr48chev - the registration says it is a '65 gmc 305e 4 speed and 4.57 rear axle with duals. Emergency brake handle is next to the shifter and actuates a brake band on the trans. gvw 10,000
    All I know.
     
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  29. Lone Star Mopar
    Joined: Nov 2, 2005
    Posts: 3,130

    Lone Star Mopar
    Member

    Deuces likes this.
  30. X-cpe
    Joined: Mar 9, 2018
    Posts: 1,215

    X-cpe

    I had a '65 C10 that I replaced the 230 in-line 6 with a 327. The front frame mounts were the same. All I did was unbolt the engine mounts from the brackets on the side of the 6 and bolt them onto the sides of the 327.
     
    Deuces likes this.

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