Outrageous Claim or Legit Car?

[DW31S]

With the VIN not on the list, the build time not consistent and other things that were pointed out, it sounds to me like the new owner is wearing some rosy glasses. As for the dealership asking the original owner to upgrade? I’d be more surprised if they DIDN’T try to sell him on the 427 swap. After all, that was a profit builder for the dealership’s sales and service dept. Then the service department tried again...no shock here either. Looks better to the GM rep if you sold the customer a shortblock rather than do one under warranty.
Nobody likes rare finds more than me, and as it is, it’s a very cool car, but until bulletproof docs can be found/viewed/scrutinized, I think the new owner is bold in his claims.