If it were me and if the one you took off the truck is an original pump, I would buy the parts and rebuild it. Good chances the shaft is scored and worn out along with the bushings and bearings in all three. The original pump, you can get parts quickly for it, not so much the the two aftermarket ones. The new shaft would be stainless steel and should last longer just sitting with water in the system.
http://www.snydersantiqueauto.com/modelaparts/waterpump