Cooling Issue - 1964 Dart GT - 273 with automatic and A/C

You need a fan shroud. A fan shroud forces air sucked by the fan to be completely run through the radiator. Without a fan shroud, the amount of air your fan sucks through the radiator is greatly diminished. That’s why going down the road with lots of air going through the radiator your engine runs cool.
You need a shroud!!!!