The non-smart a$$ answer is that there are a few problems with putting a TH350 of 700r4 behind a 235/261:

First, auto's need a flex plate, not flywheel- No big deal to get one of these.

Second, motor mounts- When you ditch the bellhousing you'll need to figure out how to mount the engine. Some outfits do make a piece that'll bolt where the bellhousing was and provide mounts. There are probably some other options too.

Third- shifter- you should be able to work this one out- I like the TH350 shifter that sticks up through the floor like the original 4-speed shifter- keeps the inside stock looking,which I like.

Fourth- You may need a bigger radiator and definately will need to install a tranny fluid cooler.

Stovebolt engine co has a kit for putting the TH behind a bolt. I've read they get it from Buffalo and you can get it from the supplier for less= you'll have to figure that out for yourself.

'51 Chevy 1/2 ton w/'62 261, HEI, offy, fentons, dual carter/webbers, t-5 & 12 bolt posi