Jim -
In the car's present condition, the only way to ever find out if it was a real Z/28 is...
1) The original POP is found.
2) The original window sticker is found.
3) The original, factory stamped block is found.
The chance of finding any of the above is low, but CRG has reunited several (blocks, POP's, etc) with the original cars over the years, so the chances are not zero.
IMO, build the car anyway you would like and have fun with it!
Paul