How can the compression be that good when the pistons are usually 0.080" in the bore and mopar cylinder heads have big chambers, like 65cc plus. I don't think that a 0.028" head gasket alone would do it.
the early pistons were not that far down, and the heads were smaller chambers, and the metal gaskets were .020. By the math it often comes to 9.2 or thereabouts
To get to 9.2, the total chamber size needs to be 79.5cc, which is pretty big actually.
Pistons .057 down is 11.2, the gasket is about 4.0, the pistons are flat tops and the heads could be up to 64.3 to still make the 9.2