I know the rarity of these blocks is what's attracting you (or any of us for that matter) but good sense dictates that if the product is true through and through the seller would let you look at it in detail. Now, if I'm the seller I would only let you look at it only After you show me the cash in your hands... Crisp and green. Only then I would think it would be a fair dealing. After you are satisfied with what you saw, and paid him, I do not think the seller is responsible for any mishap to the engine thereafter. So go visit him with money in hand, show him you're serious, and take it from there. Depends on how badly you want a (potentially) original block from 1966...
Good luck, I hope you found a true jewel
Hector