Interesting. The stamp being almost under the head is concerning, plus the existence of two VIN stampings on the block, front and side. Discounting these, it would be the #3 Z to have a Tonawanda block assembled in Flint. Most 388 blocks appeared as 350/300 hp applications, like the one in my Z11, whose stamping is more conventional in appearance than this attempt. With only two verified Z's with 388's(unless more have been documented since the CBTN was authored), the seller should have investigated better before making his declaration about the engine.
Bunch of stuff wrong with this one, although the tag looks legit (maybe replaced, or R&R'd).
Regards,
Steve