What Dave said. The thermostat sets the minimum operating temperature. If you're running too hot, and "middle of the gauge" really means nothing, then there is an issue elsewhere. Put a thermometer in the coolant and see what it is.
Very generally speaking, high running temps at low road speeds is usually an air flow issue. As a rule of thumb, if you can put a piece of paper up against the outside of the radiator and the fan holds it there it's usually ok. My 51 had an issue, turned out there was a lot of very fine dust packed onto the fins of the radiator. So while the airflow was sufficient, the heat transfer was impeded by the dirt. A good hosing out of the fins fixed it.
High running temps at highway speeds is usually a water flow issue. Check that the coolant level is good and the thermostat isn't partially closed. Sometimes the impeller on the pump comes loose or has eroded away. Wrong pulleys got installed at some point, belt is slipping, etc.
My 51 had a 4 blade fan, Chrysler, at some point, put 6 bladed fans on these engines. I don't know their reasoning but it may apply to your situation.
Of course this assumes the water jacket isn't full of crud and we all know that happens.