Dealing with this in a legal manner is not going to happen. The reason is it was used on both sides and yeah he said it was this but at the time the transaction took place you had rights to inspect it and you took the option not to. I only question the fact the balancer and flywheel should have been a dead giveaway if it was a 383 / 350 block Ect.
What this guy did was wrong 100% and is not a man of his word. I would never deal with someone I did not know without seeing it with my own eyes.
I am a fraud investigator by day... wish I had better news.
The one and only option you have is to sue in small claims court and hope to win a judgement against him. This was it cost you $50-$100.00 to file and the best part no attorney on either side.