First off you need to replace the thermostat. I had a 68 Valiant with a slant 6 in it and once I had a thermostat go bad just as you had. I removed it an put it back together and I noticed that at highway speeds it would run kinda hot. I spoke with an old guy and basically what he said is the water is going through the radiator so fast that it has no chance to cool. This made sense so I put a thermostat back in and the overheating was gone. I put a 180 degree thermostat in and it never overheated again. Once I put AC in the car I went to a 195 degree stat and it could idle in 100 degree heat with the AC on and never get over 200. Also a thermostat in the engine will help it warm up faster especially in cold weather and prevent hot and cold spots in the engine. It helps keep all of the engine at the same temp. This also helps the engine to run better. Also make sure the replacement fan has the proper pitch and that the radiator is not blocked by bugs, leaves etc. Good luck.