Put a 351-w/C-4 in my 55 sunliner and was never really that happy with it,the 351 ended up not being built to the way I wanted and never performed like I was expecting and kind of felt guilty about what I have done since my dad bought it new. I picked up a complete drivetrain from a 55 that could be dropped in and go but after driving mid 50s Olds with a hydro that FOM that starts out in second is not the trans I want to use and not wanting to get rid of the highway gears it has to use that trans. Who has a new trans behind a Y block and what do you have,I got the parts to put a FMX in it but those trans are heavy and if this car had a 3 speed manual I would have never touched it and just rebuilt the 272and went from there.
Your 55 Ford-O-Matic is a 3 speed trans. Your are correct that it starts in 2nd gear if you leave it in drive. I think in 56 they made a modification to the valve body and if you floored it the trans would shift to low and run all 3 gears. My 57 T-Bird works that way. I did pick up a Cruise-O-Matic out of a 60 T-Bird but I've never changed it out. That's because you have to pull the engine to get an auto trans out of a small bird. It seems like there is an adapter to change to a C4 and I know some T-Bird guys had installed a Ford AOD. Genstar had an adapter for a Y block/AOD, but it is pricy, over $900.00
By the way, the FMX isn't much heavier than the Ford-O-Matic you currently have. They are both cast iron.
I have an FMX behind the 312 in my 57 wagon. It was not the "direct bolt in" myth that was perpetuated by Street Rodder magazine years ago. If I was to do it again, I would overwhelmingly go with an adapter and use a C4.
I've got a 62 medium case T-Bird in my 56. It's ok but if I had it to do again I'd do a C-4 with adapter that's the bell housing. I think a Flat-O. If you do you can install a new foot feed and dump the "monkey motion" there now. For an OD I'd put in a GM 200R4.
Read post # 12 on this thread [and keep the Ford-o-matic] https://www.jalopyjournal.com/forum/threads/ford-o-matic-valve-body-change.393794/ edit: Get this out of Australia. https://dellowconversions.com.au/product/ford-v8-y-block-272-292-312-ci-to-c4-c9-c10-automatic/ The Aussie $ has dropped against the US $ [and they should credit you the "GST" sales tax for export]
I have been trying to find a 62 green dot COM that came behind a 292 to put in it but have not found one yet,I hurt the C-4 a couple years ago and needs to be rebuilt for one more trip to Florida and might just get the adapter and call it done. No sense in letting a rebuilt trans just sit around.
Mine is a green dot and I made all of the linkage work. I added to the column about an inch. The old pressure arm is now just a kick down and it is a vacuum modulator trans. I had speedo problems and torque converter problem too but the trans shop knew what to do. Too much to go into here.
I was wanting to do the 62 green dot trans with a FMX valve body and should bolt in if it came from behind a 62 292 and I do have a 62 flex plate,since it has to go to Florida one more time with the 351 and the C-4 needs gone through again I will spend the money on the adapter to use the fresh C-4. I was really wanting to do the FMX with the original air cooled FOM bellhousing so it looked like the original trans but I do not want to waste money going through two trans.
The present C-4 came from a 70s F150 and the shift arm hooked to the original linkage with no modifications.
We put a 700R4 behind a reasonably well built 312 Y block in our 49 shoebox coupe. We used a wilcap setup. Its a my wife’s car and a pleasure to drive , but stomp on it and the nose comes up and it moves.
I am going to keep it a 3 speed automatic since I recently had some new 3.00 gears put in the 8 inch.