Every Summer I have the same cooling problem with my 1975 450SLC. The car runs extremely hot at sustained highway speeds (70-75MPH) with the AC on. With the AC off, the temp is fine at highway speeds. In cooler weather, I can run at highway speeds with the AC with out any problems, but in the Summer Texas heat (100 degrees or more ambient temp.), the car wants to over heat. I have recently replaced the water pump, all hoses and thermostat (75 degree). Approximately two and a half years ago I recored the radiator. I am wondering if this is a trait of this vintage SL-SLC, and if the radiator capacity isn't large enough. I am also wondering if the capacity can be enlarged by adding an additional core or two to the radiator and still have fan clearance. I don't want to re due the radiator again and still have the same problem. Any suggestions would be greatly appreciated.