Unless my math is off, that's only 0.09" shorter than the Keith Black pistons. That doesn't seem like a lot. I wonder what the stock piston compression distance was in 1973?
Your math is right but you need to consider the actual deck height of the block. Most are at least .010 taller then the stated deck height of 9.6(9.599). I have seen blocks that were .030" taller. .010 to zero deck height is what I shoot for normally. A piston at .010" down is pretty easy to compensate for with a thinner head gasket.