What a cool idea!

The Datsun 1200 spec is 12 liter/minute @ 5 meters. Outlet is 1 inch inside diameter. It requires the maximum flow at peak HP, so it depends on how much HP you will build in, and the radiator size/efficiency.

For drag racing, you can turn off the pump while accelerating. For street use, an electric water pump decreases the reliability (because the standard mechanical pump is virtually 100% reliable).