The compression drop was not due to a change in heads it was because the compressing height of the pistons was decreased which made them ride lower in the bore.
Never said that or hinted it as far as I see it. He did say, milled heads with a .020 gasket. Considering he doesn't know for sure the piston height due to possible year change(s) from high to low ratio's.
I did tell him to measure everything up so he knows for sure and go from there. OR he can shot the target in the dark.