I agree that he should compensate you for the added benefit of having a numbers matching car, that can be anywhere between $10-20K, so you do the math here.
If he can't afford it, you can do the trade, and one day the karma will come around.
I, too am faced with the same situation with my car. I have two date coded correct HP/HP2 blocks, one complete in the car, but I'd give them both up if I could get the numbers matching block. I even came up with the idea of having a website similar to Barry T/A's but only to list 340-up blocks so that the blocks can go back to their rightful cars, but at this point I suppose a supplement to T/A's website is the best way to go.
Barry T/A, you said earlier last year you had info on my car from 1983, hmm? I'd love to get ahold of that information.