I wouldn't get all locked into getting a date coded block for a clone car. Especially one that needed some type of repair. New bare blocks are available, and with a clone, who cares about date coding? Now the other bits (heads, rocker shafts and arms, rods, crank, intake) are worth getting. You are going to use new pistons and valves anyway, and if the crank is badly scored you can them new also. Price out the parts if you bought new, then make him a reasonable offer.
If you buy a crate motor, disassemble it BEFORE you install it and get it completely checked out. Some of the problems with them I've heard (and experienced) are: leaking rear main seals, breaking rocker arm studs, misaligned ring gaps, and general internal engine cleanliness. Even if it's guarenteed, I would not want to be pulling a motor in a freshly restored car because of a problem. You can get a built hemi from FHO, Aruzza, etc...for about the same $$ with much better quality.