Thanks for any reply. Rebuilding 1976 400 has the 6x heads which are 98-101 cc which yield 7.8-1 cr also noticed pistons have a large 45* chamfer about near .200 long, these are a flat style with valve relief.  Would going to a true flat top piston with valve relief yeild more compression?   Wan't to stay with streetable power and premium fuel. thanks  

Aside from the typical TRW pistons which are basically stock replacements in a forged design, Rock&Roll Engineering has a custom design, true flattop, Sportsman Racin Piston made by JE, they cost about 30% more than TRW, but may be what you are looking for.
http://www.pontiacpower.com/catalog/pistons1.htm
Steve