Pulley size ? on cooling

I'am running a "65" 413 and it has a crank pulley of 5 1/8 stock and a aluminum water pump pulley about 6.25. Has a 180 highflow thermostat. Runs good in the country, but gets hot 210, plus in stop and go traffic. It has a 13lb radiator cap.
Put on a bigger crank pulley. U r severely under driven. Kim