I've got 75k miles on an iron head GT40 longblock I bought new. I swapped the B303 that came in it for an E303. It runs great and burns virtually no oil even after several years and all those miles. The downside is that they came with cast pistons, although that may be better in a pure street car, since they are usually tighter and quieter.
I'd check to see if it's a factory GT40 versus a rebuild -- my info is that the stock of new factory motors is long gone, which may explain the cost difference. I think new factory long block motors were twice that much. Not that there's anything wrong with a rebuild, but there is a quality control and parts quality question on any rebuild as opposed to a new factory motor, as well as the question of an overbore and machining techniques.