Hey man I would be wondering the same thing you are I would have to say I would not buy it! If he dont have any paper work proving it then no. Or ask what garage did it and ask them. You could always check this guy out.
http://stores.ebay.com/SELL-IT-INC
351w short block for $900 with 12month 12000 mile warrentay. they also have 302s 331s long block short blocks this is where im going to buy my engine