The 671 heads are a real good head, and will increase your compression ratio because of the smaller combustion chamber.
Cast Year CID Horse Power Valve Size CC Application
671 1967 400 350 2:11/1.77 72
671 1967 428 360, 376 2.11/1.77 72 Early Grand Prix 428 H.O.
Static compression, (C.R.) on a Pontiac is a function of the chamber volume and the engine displacement. If the chamber volume is increased, the C.R. goes down; if the displacement is increased, the C.R. goes up. You should be over 9 to 1 on your compression ratio though, but it should put out a bunch of power.
How much does he want for the heads?