As far as the gas smell....well that's part of it. He may not have known and that doesn't mean it wasn't fresh.
As far as the crank being a 350 well that stinks. Is it a 400 block or a 350 block? I think a .060 over 400 block with a 350 crank still gives you a 383...It is a shame you can't trust a mans word but truth is most times you cannot ....I'm sorry to be blunt but I have a hard time feeling sorry for you if you did not pull the pan and look before you traded...especially since you were gona pull it and check it out anyway(proven by the fact that you pulled the pan at a later time) Don't play russian roulette then complain when you shoot yourself in the head.
I realize you were not asking for sympathy with this post so don't take that comment wrong.