I have a new engine and would like to test run it before putting in the boat. I need to run the pump but don't have any cylinders to connect it to. Has anyone run the pressure to the return side with a valve. any suggestions

Why not just take off the belt and not turn the pump?

Exactly what I was thinking if he has V belts. If its a serpentine belt and has a recirc pump, then you pretty much have to run the belt.