The Buick 700R-4 will most likely have the BOP (Buick, Olds, Pontiac) bolt patern. If it does you will need an adapter to put it behind a Chevymotor.

A used automatic transmission and converter out of a RUNNING doner car can be a good deal. Same out of a wrecked vehicle. But a transmission out of a yard with no history is a core - just a place to start rebuilding.