CHAMPION PLUG HEAT RANGES

So colder is better for a performance engine? I am a good mechanic and can do just about anything on a car, but I don't know much about spark plug heat ranges. I have a 69 Cuda with a 340 built to about 375 HP with 3.55 gears and a 4 speed. what heat range would be best? I usually use Autolite plugs, but I guess I could try Champions.