Just from common sense, wouldn't the NA piston have a higher compression ratio anyway? Most forced induction engines have very low NA compression ratios as they get compression from the induction systems.
The other thing I would be worried about, aside from the difference in compression, is the difference in weight. I would recommend to anyone who does this sort of thing, to weigh each piston, rod, wristpin etc so you have a balanced rotating assy.
I agree with the others - get a used shortblock.
