One could ascertain to 'his own satisfaction', that the car was (or was not) originally a Z28 by careful checking of *everything else*... ie. if a car under warranty received a replacement short block via a dealer, then generally ONLY the short block was replaced, so the heads, intake, carburetor (and all other parts installed on the engine) were simply re-installed on the new block. Does the engine *sound/run* like a 302 should; a Z28/302 short block replaced in a Z28 during warranty would be *correctly* built per Z28 specs by the Flint assembly. Is the casting number on the block correct for a Z28? Is the casting date on the block consistent with the PERIOD of the warranty on a '68 Z28? If the cast date on the CE block is before the car was built, then it is not a warranty CE block *for your car*. If the cast date on the CE block is significantly AFTER the warranty period for the '68 Z28 then it is likely NOT a warranty CE block for your car. Check the other parts in the engine compartment like booster, MC, etc
* So check all those parts to ensure that those parts are a) correct for a '68 Z28, and b) dated consistently with the VIN of the automobile (the orig short block being gone prohibits date checking based on the engine)...
* Check the 'other parts' not associated with the replaced short block to ensure they are correct for a Z28 and dated consistently with the car's VIN.. ie. bell housing PN, transmission (should have a VIN to match the car and be a Muncie 4spd installed on a Z28 in '68), rear end (should be a 12 bolt probably BU? dated consistent with the car's VIN production date. Check the springs, brakes, *everything* that you can to see whether they are consistent with Z28 production.
* Are there parts installed on the car that are NOT possible for Z28, and they appear to be 'factory installed', then that would be a negative...
IF I checked all those things, and the parts/casting nos/ dates/etc were 'good' for 75-90% of the Z28 parts, then I would feel pretty confident that the car WAS in fact a Z28 from the factory, AND if I wanted a Z28 for myself, to drive, then I would pursue it. If you only want a Z28 for *resale*, OR if you don't really want a Z28 at all, then do not pursue it.