Did you ever get your overheating problem resolved? If so, what was the root cause?
My 68 GTO 400 is exhibiting the same symptoms, with the exception of it over heating when outside temps are 70F or less.
This is what mine does, overheating only when A/C is ON or OFF but, worse with the A/C ON and driving at freeway speeds.
When I get off the freeway, the engine temp comes down to normal.
I have a 160 degree thermostat, I can’t give you the exact temps because I’m going by the factory gauge. I know it runs hotter under the above mention conditions because it starts to spark knock and it will spit up a little coolant at the over flow hose when I shut it OFF. I know the fan clutch, needs replacing but the fan clutch should have no effect when I’m on the freeway because of the airflow at 70+ MPH across the rad and condenser. I’ve watched the coolant flowing through the radiator when the T-state opens, it flows very fast through all the cores and it even looks like it foams up a little because of the fast flowing coolant. The heater core is by-passed, and the radiator is a 4 core that has been re-cored about 5 or 6 years ago.
Any help is appreciated, Thanks in advance
Frank