Over heating 66 Dart

The way I filled the radiator was to put about a gallon of pure coolant in the engine (cold) and a gallon of water. Then I would start the engine and begin to fill the radiator until it was about an inch above the rows in the radiator. I would keep the engine running until it came up to temperature and fill the radiator with 50/50 mix until it remains at about an inch above the rows then put on the cap. Thanks