You used to be able to buy a bare cast iron block from the dealer with the number boss blank. In today's world it would be nothing to duplicate the factory numbers on the block. And I don't think anyone is going to do an acid test even on a block that is shaved and renumbered.
If you go to sell at an auction, they do some authenticating but they aren't going to go to the extent of an acid test. You have to sign that "to your knowledge" that the numbers are authentic. I doubt any collectors go to the trouble either.
Now, the buyer of this POS might eventually either establish that the numbers match or make them so. All of that being said, it is my learning that GM did not keep records as Porsche did in this time period, i.e, the number of the chassis and the number of the engine installed in that chassis. GM,
AFAIK, can tell you what year a motor is but not what specific chassis it went in, just the model. If that is in fact the case, it would be a lot easier to provide a correct block legitimately for a lot of models.
For the rarer production units it becomes more difficult. This Corvette is just a production line model with a no name motor other than being a '65 or '66 casting by the casting number and possibly followed by a "Y" back that early. Body serial numbers began appearing about 3 years later. I'm pretty sure there is a way to determine if a particular chassis was a 327 or a 427 but in '66 getting a 327 to match does not seem to be a big deal.
350 CI engines came out in '67 but the '66 Vette could be had with the new 427. We didn't see the 454 until '70.