for one, that is assuming its on the lower end of manufacturing tolerances, stock compression was suppose to be 9.1:1, so I've read, although I don't doubt quite a few stangs out there are running 9.5:1, as quite a few E7s are cc'ed to be 58cc.
numero dos, 73-76 302 block is somewhere in the ballpark of .023 taller, lowers the compression.
I'm not sure of exact numbers on deck height, but a 58cc chamber + 12cc dish in the pistons still should be around 8.5:1, when in actuallity, its 8:1, the block is higher, how much so, I don't remember.