[MagicRat]

You may be aware that there are 2 differnet GM V8 bell housing bolt patterns; one for Chevy small and big blocks, and one for all other V8's, including Buick - Olds - Pontiac - Cadillac. (BOP pattern)

By far the most difficult problem you will have is finding a 4x4 auto transmission with a BOP bolt pattern. Your existing trans will not fit the Caddy engine, and obviously no 4x4 trucks came with
Buick - Olds - Pontiac - Cadillac engines.

There is only one possibility I know of..... the late '60's and early '70 Jeep 4x4 Commando's came with Buick 225 cid V6 engines.....and some came with a Buick 350 V8. The trans and transfer case from one would fit the Caddy V8 and may be adapted to the Chevy truck .

If your truck was a 2 wheel drive, the swap would be easy; you would just need to find an ordinary THM 400 trans with a BOP bolt pattern and lengthen/modify your driveshaft to fit.