Under or Over

I am going to buy a bracket, Pulley, and Alt. kit for my 408 from CVF...For the water pump pulley they offer a under drive or high flow option for the water pump pulley. So my question is do i want to slow down the water flow and keep it in the radiator longer or increase the flow? I am running this combo in a 66 Dart and have read on this forum about some guys having overheating problems in the early A,s with this engine combo....Thoughts??