1955 60 Special Overheating

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if the car overheats at idle only, it is moslty an issue of not enough airflow to cool the radiator. Especially if the radiator is very hot and the fins are very hot (same temp as the block). try measuring the temperature of the fins at the top of the radiator using an infra-red thermometer.
if water boils over in the radiator it means that it is heated properly by the block, therefore the heat exchange is taking place between the water and the block.

Also, It could be that the ignition timing is set too late, therefore the block is generating too much heat which cannot be cooled down by the little airfllow using the fan only.

try advancing the timing to the max and disconneting the vacuum advance tube.
In the distributor, the springs that hold the distributor sometimes become weak over the years, therefore the vacuum advance is too strong and advances timing too much when the gas pedal is depressed and air is sucked in the distributor. This leads to a tendency to set the timing too late to avoid detonation when depressing the gas pedal.

Rob

Some different ideas... I had a car that was bad overheating due to a lean fuel mixture, that however was at higher speeds in my case. Check intake manifold torque. With it at idle... spray a mist of carb spray into the carb...if it picks up...richen of your mixture screws might be needed but first check for vacuum leaks with the carb spray, engine running. Spray short blasts of carb spray at possible vacuum leaks, manifold, carb base ect. if the engine picks up in an area...check for vacuum leaks.  Check your vacuum advance/point dwell & idle speed. Vacuum leaks will cause a poor mixture. I usually put as much advance as the car will take on the timing...then back off to surpress any ping. Timing chain too loose, jumped???  Remove the dist cap and view the rotor as you rock the crank by hand  back and forth to gauge how much slack you have.  Upgrade your fan, GM did on later models. I found 'Cascade' dishwashing powder an EXCELLENT (the BEST!) cooling system flush EVER. Flushing a cooling system does put at risk your waterpump. Check that the exhaust heat risor is fully open. Any other exhaust restrictions?