350 through 455 heads will interchange on 350-455 blocks. Heads with large valve will be problematic on 350 blocks due to the bore being smaller
causing clearance issues and valve shrouding (350 only)
455 heads have larger chambers so putting them on a 400 would lower compression, causing some loss of performance
I hope you have a 455 as that would be an street motor (lots of power) that could be had with minimal funds
A 400 is a great engine and can be stroked to 462. They can be made to run like a striped *** ape.
Remember a pontiac allready has a 3.75 inch stroke , those chevy guys spend big bucks and time to stroke their 383 motors. 400 poncho is allready there. and has a stronger bottom end. onnecting rods are the weak link, but decent forgings are readily available for very little $$$.
Plenty of stuff around to make the pontiac a real runner. Stick around here, and ask questions.There is lots of help here for you. LA