What About Edelbrock 340 Heads?

yada yada yada on Quinch. My 408 with flat-top piston ran low 9.70's@2860 pounds in my Duster, 10.08@3200 pounds in my Sons Duster. Now they are on his stock crank flat-top pistons combo running 6.54@3200 pounds. That's around 10.30's or better in the 1/4. No tricks, no bells, and zero whistles. Ohhh and neither decks were cut on the blocks.
Flat top pistons are sitting where in the block? With how thick a gasket?
Thanks.