I didn't get around to looking at my new one with the BBC this weekend. Worse case scenario, can you cut the shaft down on one of your other engines you were looking at using?
I've actually done this once before but it was only a half inch, and there was still plenty of thread in the tapped hole. In this case, a 3.5 inch shaft will not have threads that go up that far.
Did the drive pulley have a set screw? Can you use a long straight slot/key for both the drive and the blade clutch?
If I can figure out a way to repower mine, I'll send you my old tecumseh for the cost of shipping.