It was common for open track Mustangs to underdrive the water pump a little to avoid high rpm cavitation. Cavitation is worse than low rpm loss of coolant flow. At high rpm the volume is going to be adequate if the system isn't cavitating. Meziere water pumps have been used by a lot of people in the 5.0 community with good success. You have to find a pump that is going to offer the flow and pressure needed to keep things cool. The drag pumps aren't going to cut it. I used to run a Moroso setup on a 67 GTO I bracket raced. Was really nice to be able to run the pump and fan while the car sat in the pits, but it would never have kept things cool if I drove the car on the street.