i thought the 77 Grand Prix had a 305 or 350? Thought only the the T/A got the 400 in 77? Thats an odd combination for a Chevy truck, Pontiac 400 and turbo 400. The BOP TH400 won't bolt to a chevy engine so if you're going to use the TH400 out of it you'll likely be stuck with using a BOP engine. Thats unless its got a Chevy in the Pontiac.

BOP = Buick, Oldsmobile, Pontiac.