Mustang II overheating bad?
Replace the bottom hose.
The top of the radiator is hot, while the bottom of the radiator is cold. That means the water is not flowing properly.
Blown headgasket, radiator too small, shroud broken, etc, ad infinitum, do not explain the radiator thing.
You say that after five miles of open road driving and it overheats. Well, its the bottom radiator hose. It's collapsing when the engine revs harder causing the water pump to pump harder but that just increases the vacuum inside the bottom hose caused by the engine trying to get coolant.
I reckon the interior lining of the bottom hose has come away from the rubber and is collapsing when the speed of the water flow increases when the engine is revved up.
I will guarantee that the times when it runs in the normal range is the days that you are not driving on open road but in the suburbs and not having to rev the engine higher.
Do this simple test:
Let the engine idle until its warm (you want the thermostat open). Take the radiator cap off and look in. You should see the water flowing past the opening. Rev the engine up to about 3000rpm (about the rpm you would be at while driving on the open road) while you watch the water. You should see an increase in speed (of the water). If not, then you have a problem with the bottom hose intake. Replace it.