i just got a th370 out of an old catalina (late 60s i think) for free. i was looking for a th400 but free is free. i know the cases are the same and i could gut a 400 and put it in. im running a big block pontiac engine and its hard to find poncho bolt pattern cases (V in the top of the bell not a ^). should i swap a 400 into it or is the 375 plenty enough for me. my engine has more torque then HP, guessed at by a frend of mine at " well over 650 ftlbs at 3000" HP is about 500ish or less. money is hard to come by even with 2 jobs, that whole wedding stuff to buy thing puts a cramp in things.

when i say swap the 400 in i mean the th400 guts into the th375 case

no dude the 375 is a th350 with a higher torque capacity.

From www.nastyz28.com ...

THM375 - This transmission is a derivative of the THM400 sharing the external appearance of the 400. It'll be marked 375-THM on the bottom of the tailshaft housing. It's a light duty version of the THM400, has ABSOLUTELY NO RELATION to the THM375B, and was found in 1972 to 1976 full-size cars. The Turbo 375 was modified to accept the Turbo 350 driveshaft yoke.

I pulled one of these out of a '72 Catalina. Just like it says above, a 400 that takes a 350 yoke. I've never heard of a thm375b, can't help with that.

The 375 is internally identical to a 400. The only difference is the output shaft; it has the 35o/PG output spline. It is so much the same internally, that one time when I needed a 400 for my land yacht, my little brother had one of his fellow techs at the Chevy dealer where he worked, to build one for me; when I went to pick it up, I took one look at it and told him it wasn't a 400; he swore it was, showed me the box from the kit he had used and everything, it wasn't until I had him go get a driveshaft out of a 400 vehicle and try to put it into the trans that he believed me.

There is no such thing as a "big block Pontiac" (or a "small block" one either). They're all the same bore spacing.

It would take about 525 CID to get 650 ft-lbs of torque. Torque is a function of the number of gasoline molecules burned per engine revolution. THe gasoline molecules do not look around themselves to see what brand of cast-iron they are burning in, and respond to the cast-iron's owner's fondness for that particular brand, and exert more or less force accordingly. The notion that one brand of cast-iron produce significantly more torque per cubic inch is laughable. If you have a 455, you are probably producing in the range of 550-575 ft-lbs of torque at the engine's peak (where it reaches its highest volumetric efficiency); which, with a big motor like that and small ports that a Pontiac head has, would be way down low like 3000 RPM.

BOP 400s are extremely common. Far more common than Chevrolet ones. For every Chevy 400 case in the world, there are (or were) at least 2 BOP cases. Maybe lots of them have got melted down over the years, since nobody cares about those motors any more; but you used to pay a high premium at the boneyard for a Chevy 400 instead of a BOP one.