Last Summer I was having an issue with the temperature running hot on my Temperature gauge. It was running in the 3/4 of the gauges range. I had replaced the non working Temperature sensor earlier in the spring. I replaced the thermostat with a 160. This did nothing to correct the issue. When I turn the cab heater on high, I can get the temperature to come down to a more reasonable value on the Temperature gauge.
The truck only has 52 K miles on it. It has sat in a garage every winter for all 35 years. When I dumped the coolant to replace the thermostat the coolant was a nice green colour not rusty. I have owned it 1.5 years, I bought it from the original owner. 302 2WD Auto transmission.
Question, is it time to replace the radiator or maybe the water pump?

 

In general, the T stat only regulates minimum temp, not maximum. If it is running warm... warm being defined as over 200-205 F changing the T stat won't help. A factory stat that opens around 190-195F is fine and helps keep the engine clean and happy. I'd suggest 180F as a minimum stat temp.

It it has an older or original radiator, that would be the most likely problem... especially if it is warm/hot while rolling. Generally, warm on a roll is a radiator problem and warm at stop only may be a fan problem. Turning the heater on to drop the temp is a clue that the radiator is not fully doing its job.

My dent was running a bit warm and also had some patched top tank radiator leaks. A new radiator fixed it. The 35 YO original was tired. I upgraded from a "standard cooling" 2 core to and "extra cooling" 3 core from AZ in copper/brass ('78 F250). Cost was reasonable.

The water pump, if not leaking, is probably not you problem unless the pump vanes have rusted off (unusual).

 

Second the worn out radiator idea. If it sat 35 years with mostly the same coolant, then you need a new radiator. Mine had 68k miles and sat. Was doing the same as yours. Look inside your radiator cap. Mine was scaled up and rusted bad but the coolant was clear/green. You could get it re-cored if the rest is in good shape.