Either a standard parts store water pump or you can look into the MP pump that has an 8 blade rotor instead of a 6, still has ports for heater hoses and all that. There is also a "racing" w/p that MP sells but it's for one of their timing covers on a newer R or P block.
I purchased a waterpump from Evans years ago. It has a 3/4 shaft and rollerbearing which is better than a stock 5/8 bearing. Also there waterpump is a vane style instead of a impeller which reduces cavitation. Jayson http://www.evanscooling.com/