I have an 84 Regal with a new well built '71 455. It's built for high performance(around 400hp,485lb-ft). Reworked heads, edelbrock intake, street demon carb, 2inch headers, alum pistons,and all the rest of the works. All of the cooling system is brand new and still manages to overheat in about 15-20 minutes of driving. 2 core alum radiator, new water pump, tried three thermostats and it's extremely aggravating. Would anyone have any suggestions?

Have you checked if you fan work ?

first thing, i wouldnt run any thermostat in it for summer driving. secondly, i agree with the fan issue. do you have electric fans of flex/solid blade fans?

I never had a problem running a thermostat in my 70' 455 but I would definately check out the fan. Mechanical clutch fans, the clutch can quit working. Flex fans don't work well, especiallywith marginal 2 core radiators. Electrical fans will have to be especially large for a 455. If it heats up at highway speeds, the fan would not be the problem. Low speeds would be. Hope that helps! jerry at www.freeengineinfo.com