I've always heard the area from in front of the engine mounts to behind the transmission cross-member. Having said that, however, I've always boxed the entire frame, just 'cause I believe in overkill !
Because I'm doing the truck in sections so I don't have the whole frame exposed. Also since I don't really need the whole thing boxed and the price of steel is so high, I'll just do the sections I need. I'm doing the rear portion now. So far I have about 1.5 feet in front of the rear axle and about a foot aft of the rear axle.
I'd have to agree with the other folks here. Box as much as you can, although I understand you're doing it in stages. Something to consider: the areas you box will be stronger than the areas you don't. Sounds stupid simple I know... but what i'm getting at is don't box everything EXCEPT for the spring mount areas, steering box area, etc. I've seen stress cracks happen there on partially boxed frames. Any torsional stress will be directed towards the weakest part of the frame, which is why certain makes and models of trucks and cars seem to always crack in one specific area or another. If anything, box the part of the frame you think will get the most stress, then fill in the rest as you progress with your project. Just my 2 cents. Good luck with your pickup.
ive built the chassis on at least a dozen 47-54 chevy trucks and never seen a cracked frame.the only boxing that ive done is the plates in the must 11 and engine mount area. i dont get it. but i guess you can build it any way you see fit.
The Jalopy Journal
