318 is not hot enough

Change the thermostat.

If your engine never gets hotter than 150, and the thermostat is a 180, then if the thermostat is working the radiator never even comes into play, since the t- stat wouldn't be open. There's no way your engine runs at 150 with a closed thermostat (no water circulating in the radiator).

So, either the temp gauge is wrong, or the thermostat is stuck open. For an easy check, take the cap of the radiator when it's cold and start the engine. If you immediately see water circulating in the radiator, the t-stat is stuck open.

Otherwise, you could get an infrared temp sensor, harbor freight sells them for like $30. Check the temperature of the water pump to engine hose while the engine is running, and compare it to the radiator. If the t-stat is working, the engine temp should get to 180 before the radiator temp changes significantly. The temperature the thermostat opens at should be pretty obvious, the engine temp and radiator temp will equalize when it does.

Nice thing about the sensor is that it will tell you if your gauge is accurate and if the thermostat is working without having to take anything apart.