If you know some history on your car such as where geographically it has resided most of it existence?? Example you have proof that it was bought and sold in the same state where you live and the dude selling your alegally original block is located in a relatively close distance as well the fact he doesn't know your Vin I would tend to believe it's yours. Maybe clear up the question....did you tell him the Vin number ?? If not you should buy a lottery ticket on the way over to pick up your # matching block. Just my thoughts.
Good luck.