I’ve got a 340 project I’m building for the street. It’s a 4.00”stroke with 4.080 bore and aluminum Edelbrock heads. I can go with the felpro .050 gasket and get 10.27 or the thinner .030 Cometic and get 10.45. The cam is a custom ground Bullet cam that will have about 14” of vacuum for power brakes. The pistons are race techs with a reverse dome and -.004 deck height.

What is your local altitude and available octane?

Those are closed chamber heads. With those pistons are you saying they are .004 in the hole at the top of the piston? .034 is playing it really close. What about a .040 gasket? A 10.4 comp ratio would work nicely.

the difference between 10.27 or 10.45 is nothing pick one and use it.

I'm wondering the same thing , if it is .004 in the hole I would have no issue running .034 squish/quench ... much better than .054. We ran the Enginemasters build I think at .030/.035 and the only contact was from the piston that had the center crater in due to the air trapped in the cylinder heads ... we ran an Indy reverse flow cooling system and didn't have any type of bleeder in the top of the heads to deal with this on the dyno.

without the cam specs and your intake centerline I would go for the higher compression. 2 tenths of a point won't really be noticeable and with an aluminum head your comparative CR will be .75 to a point lower anyway. you should still be able to run 10.5:1 even with an Iron head.