The cylinder heads have different size chambers machined into them. Pontiacs have machined as opposed to cast chambers, and the 4bbl heads had less depth to them. I'm using 1bbl heads in all cases.
The advertised compression was 9:1 on the 1bbl and 10.5:1 on the 4bbl, but every person I've heard that did actual measuring (including myself) show the compressions closer to 8.0:1 and 9.5:1. So something is amiss between the advertised spec and what has been measured. At full lift the valves don't extend past the head deck very far and the piston valve relief's are way over kill so I'll probably make up some compression when I go to a custom lightweight piston. I don't have the cc's on the old forged pistons to compare to the OE cast ones.
