My 1989 GMC Sierra (350, 2WD) was leaking from the water pump and overheating...Installed new water pump and serpentine belt (after getting it off, it needed to be replaced)...Truck is still overheating, none of the hoses are warm once truck is running..Did I install incorrectly?

When replacing coolant, did you bleed the air from system! you can remove radiator or overflow cap when cold. start vehicle and let it run until thermostat opens,when all air is out hoses should get hot. but before starting, turn heater on to high setting, if you vehicle does not have a air bleeder on it. do as above. just be sure radiator is full of coolant. after hoses get hot, turn off heater, then put cap back on radiator or overflow. good-day! the first thing to do is to check for a stuck closed thermostat. do this first ok!