QWK just answered the voltage question.
as for grounding the pump, i dont want to give you wrong information, but from memory on the S chassis i took the ground wire from the pump, and connected it directly to the chassis of the car.
this took out the 2 speed switching circuit.
youve already mentioned a relay, if you hook up the pump directly to the battery with new wiring, youll be laughing. also allows more stable voltages if/when you upgrade the pump.
dont use the amp wiring to power the pump. give it its own wire.