thermostat is stuck closed
Overheating and still coolant in resivor. Do I have to
change the waterpump
If you know your thermostat is stuck closed wouldn't you first change it out and see what happens? The water pump provides the circulation of the coolant through the engine, heater and radiator. If it was bad you would see no obvious flow in the radiator neck but, the thermostat would need to be open.
Before putting a lot of money in this vehicle have the coolant tested for combustion gases. Northstar engines are notorious for blowing head gaskets and if you have combustion gases in the coolant you have a blown head gasket and the vehicle is not worth fixing. Hope that helps! Jim