The Chevy bolt pattern is different than Olds, Pontiac, Buick , and Cadillac.
Like the TH700R4 overdrive was only made to fit a Chevy. The TH350/400 was made for Chevy, or BOP (Buick-Olds_pontiac) but also Cadillac. Don't know about swapping the case on your 4X4 trans. Some transmission shop may have the answer to that
BILL
I just remembered.... Flashcraft has an adapter for bolting the Caddy to a Chevy/GMC transmission. According to my cataog, they have an adapter.
You can get on their site at
www.cad500parts.com The price in my old catalog is 129 bucks. includes bolt kit etc.