[Flathead Fever]

I would start with an infrared thermometer gun and measure the outside temperatures of the engine's coolant inlets and outlets on the radiator. Then check the temperature of the radiator's core in different spots. Maybe even the exhaust manifold ports temperatures. At work most of the time overheating was a radiator or thermostat but, on an engine, as old as a flathead it can be all kinds of different stuff. You can take a block pressure test, put some pressure to the radiator with a leak tester, not too much pressure or you might blow up a soldered joint on the radiator. Let it sit for a long time and see if the coolant system holds pressure, if it doesn't hold then pull the plugs and see if any water squirts out of a cylinder. Usually, the leaking cylinders spark plug will be washed shiny clean, no carbon like the others plugs would have. Head gaskets eventually fail and yours are over 20-years old. One reason is some people refuge to use coolant in their engines but it's not there for only cooling it prevents the gaskets from rusting away. Really important to run a 50/50 mix of coolant.