1959 Other Ford Models Overheating

Engine Cooling problem
1959 Other Ford Models V8 Two Wheel Drive Automatic

I own a restored 1959 Ford Skyliner. It has a 390 CU and is an automatic. I have a new 4 core radiator, a flow through thermostat, a new 5 blade cooling fan blade, a fan shroud was installed and is about 50/50 water anti freeze. The car has air conditioning with a condenser in front of the radiator about the same size as the radiator. The car runs fine when traveling but if I get stuck in traffic or parade cruise it will get hot to the boiling point and run over. I must stop and cool it down. I did all I could except install an aux electric fan but have little room to install it. Why does this car overheat in slow traffic. I'm at my wits end.
Thanks.
Ken