318 overheating

I thought I'd throw out another idea. The electric fan is it pushing hot air from the engine side of the radiator into it or is it pulling cooler air from in front of the radiator toward the engine? Many of these fan kits can be wired to spin the electric motor in either clockwise or counterclockwise rotation. If the engine stays cooler longer when moving you are receiving the benefit of air being forced across the cooling fins even as the fan may be fighting against it. When not moving, the fan may be forcing nothing but engine heated air into the cooling fins. With out starting the car, hotwire the fan so it runs and see if your drawing cool air from the front of the vehicle if your fan is mounted on the engine side of the radiator or pushing cool air through the radiator if mounted on the grill side of the radiator. You should be able to feel the direction of air flow easily. If you find this is a problem simply reverse the two wires coming out of the fan motor to get the fan to spin the opposite direction. This came up very recently on a '48 willys with a SB chevy that came into the shop. Owner had installed a new aluminum radiator and electric fan and ended up with worse cooling situation that when they had a half plugged radiator and a pump driven fan. Owner had followed installation instruction to the letter but they did not say anything about reversing rotation when changing the fan mounting from front to back of the radiator. Darn Chinese!!??