how much compression for pump gas

"Quench" and "X heads" cannot be used in the same sentence unless the word "no" preceeds the quench part...:D.
The probelm is, the chamber design and shape prevents any quenching areas from forming, unless things are modified and the right piston is used. Flat tops are not the right piston. Do you have the cam card or part number handy? I need the intake valve closing point @ .050 lift. I'm showing around 9.5:1 with the std Fel Pro gaskets now. I took a stab at the intake closing, I put it around 41° ABDC, and that's coming up with 8.1 dynamic. You are right on the edge of detonation now using those numbers. The time to design the engine to maximize quench has passed. You can't get any with what you now have to use. So I would ignore it for now. The best deal would have been to build the shortblock with the pistons set at zero deck. Then later you're a head change away from a tight quench and better heads. With them .020 down, you will need special gaskets, and special ultra smooth deck finish to run them, just to get a decent quench.