Well, a 200r4 should have a universal bolt pattern and bolt up to a poncho. However, its lifespan would be quite dependant on how the trans was (re)built and the torque put out by your pontiac. The 400 could put a 200r4 in a world of hurt in short order if it was stout and the trans was stock. There are adapter kits for the 700 4r available from places like Summit, and the 700s tend to stand up but a lot of that depends on how it was (re)built too.
This can be done without too much extra hassle over a sbc so don't get intimidated.
