Need some more clarification Robert. You posted a question about the gauge in the truck but give temp readings from what appears to be a hand held temp reader.
What type of gauge is in your dash panel and whats the temp reading on it when your testing?
What is the thermostat's temp rating, i.e. is it a 160, 180 or higher degree thermostat?
The way the heater circuit is plumbed into a Stovebolt 6 engine some of the coolant is circulated through the heater core where it's cooled by releasing it's heat into your cab and the cooler coolant is returned to the water pump and recirculated in the engine. A heater core is basically a small radiator....
With cold outside temps this may be enough cooling to keep the coolant below your thermostat's fully opening temp. Since your getting 160 reading at the upper radiator hose the thermostat is atleast partially opening but may not be fully opening. This would be a OK if that is whats happening.
160 degrees at the upper radiator hose, good heat in your cab,no coolant leaks and 180 or less on your dash temp gauge = a good running Stovebolt 6 engine.
