Don't know much about your bus but GMC's did run Pontiac engines. No 400 in that year though but that motor should make it a pretty strong runner. (For tech info on Pontiac engines go to the Performance Years forum.
http://forums.performanceyears.com/forums/ ) It would be a nice project to restore. You'd certainly be the only one at the car show! Give us updates if you work on it or sell it.
