My brother just had the heads off his stock, 53k original mile, 1972 318 about a month ago. The below deck height of his pistons measured at 0.070", but I've read that a stock 318 can be as far as 0.090" in the hole.

Assuming 0.070" in the hole, if you just go to a 0.028" MP head gasket you would get 8.97:1 compression. For ever 1cc you remove from the chamber you will gain about 0.1 in compression ratio. What that translates into for how much you have to mill off your heads I don't know exactly, but it should be about 0.005-0.006". Thus to gain another half point in compression ratio you are going to need to remove about 0.030" from the heads.