1972 Demon 340 engine mods

OK I checked what it cranks out and its a low compression for sure as it cranks 125 psi all around.

Now its probably close to 8.5:1 so small chamber heads will bring it up to around 9.5:1... Thin gasket would probably make it closer to 10:1 but no quench or swirl advantage of aluminum heads what so ever...
What is standard J head chamber size? Is it around 73cc? Also what is the thinnest gasket I can use to bump compression as high as possible?
I will take the heads off and cc the piston in tdc to see what exactly this thing will be.

It is what it is...