If the temperature is creeping up at idle, it's typically caused by insufficient coolant flow, insufficient air flow, lean air/fuel mixture or timing. It can also be due to a combustion chamber leak that builds heat and pressure in the cooling system.
First of all, make sure there are no combustion leaks.
If all is healthy, then verify timing and air/fuel mixture because they both will create excessive heat that the cooling system cannot remove.
Because your problem is limited to idle operation, you want to verify that your A/F mixture is not too lean at idle AND that the base timing is correct. If you are running ported vacuum for your vacuum advance system, you could experiment with manifold vacuum. Advancing the timing at idle will help it run cooler at idle, but it can lead to part-throttle acceleration stumbles when the throttle plates open. It's been my experience that experimentation is the best way to know if your engine will like to run with manifold vacuum for the advance function.
If all those items are where they need to be, you're back to confirming air flow through the radiator at idle. You don't mention a shroud, but I assume you have one on the car. If not, add a good fitting shroud. Make sure the fan shroud fits properly with no gaps around the radiator that allow air to sneak around the perimeter of the radiator instead of being pulled through the fins.
Also, make sure the fan is positioned properly in the shroud; the blade tips should be 1/2 to 2/3 in the shroud.
The fan diameter should be correct for the shroud opening; the blade diameter should be about 1" to 2" smaller than the diameter of the shroud opening. And of course, verify that the fan pitch is correct for the engine rotation (though if that was wrong, you would have problems all the time).
If you're running a high compression combination with cast iron heads, you might end up needing the aluminum radiator because the cooling system can't handle the heat generated by the engine. BUT, since you don't have a temperature problem under normal driving conditions, you should be able to solve this without needing an aluminum radiator.
I see you also posted on the PY forum. There is an excellent reply there from OldGoat67; his comments are "on the money" as far as the cooling system for stock engines.
1968 Pontiac GTO
1983 Pontiac Bonneville (G) wagon
2008 Pontiac G8 base