A couple of years ago, I was having the same problem with my '69 RS, base 327, non-AC car. When I rebuilt the engine, did a few minor tweaks, and the 4 blade fan just couldn't provide enough air flow in slow traffic or long idle situation.
Changed to the 7 blade fan and clutch and the problem was solved. I have the standard, non-AC radiator.
Just this past weekend, Hot August Nights in Reno, participated in the cruises and no issues. Abient air temps during the day were in the low 100's.
If you aren't hung up on maintaining the original 4 blade fan, the 7 blade fan and a clutch would be my suggestion - also be sure you have the shroud properly installed. I believe your AC equipped car should also have a sealing strip between the top of the core support and the radiator to reduce leakage around the radiator.