[Richard Knight]

As written by the O.P. the I.D. of the pump hub is binding on the cast iron nose of the pump. As pointed out by swamp ledge there are clear instructions. It is not just the fan I.D.,the O.D.of the pump nose is often larger than the original pumps. The solution is simple, grind away the offending cast iron. After that I think you will find the fan will go on the taper just fine.