I got a Buick 350 in my 69 Chevy SWB Step Side. Its 2WD so I can't help you on the transfer case side. Motor mounts I fabricated from anle iron, square tube, and plate steel. Not a big deal if you can weld or have them welded.
I do know that the trans bolt up is different on the Buick, Olds, and Pontiac....but basically the rest of the trans is the same. Perhaps you can take the tail housing from your current 350 and tranfer it to a B.O.P. trans?
My Buick 350 is a runnin' SOB ! !
Give it a try....I did and don't regret it other than the "higher cost" of Buick V-8 parts !