I wouldn't think that the shaft size would matter on the water pump. as long as the end of the shaft that the fan fits on is the same. just make sure the other dimensions are the same, and if all else fails, get em both, and bring back the one you don't use.
they have different shaft heights for different pulleys ,,, you need an Airtex 895,,,and that should be the 4 3/8" shaft,,,,,but take yours off,,,see how much the flange sticks out from the base,