cast iron water pump vs aluminum pump

Cast Iron Water Pump: diameter of fan pulley is 6 1/2" and crank pulley is 6 3/4"(original stock pulleys for sixties 318, 340)
Aluminum Water Pump: diameter of fan pulley is 6 3/4" and crank pulley is 7 1/4"(original stock pulleys for seventies 318, 340)
If you decide to change to an aluminum water pump you will also need to change the alternator bracket, if you have power steering, that bracket also, otherwise none for the pulleys will line up correctly, not only does 70 and up pulleys have a larger diameter they also have a different offset they are deeper or taller. The v-belts might need to be changed, they probably won't be long enough.
Milodon makes a high flow mechanical cast iron water pump part #16251