The plug does not have anything to do with how hot the combustion process runs. A hot plug cannot burn a piston - that's nonsense. A "hot" plug has a longer heat path from the electrode to the body of the plug, so the tip of the plug stays hotter to avoid carbon fouling. It has to do with the running temp of the plug itself. A cool plug dissipates its heat faster, so the tip of the plug is a little cooler. It will not affect engine temperature or combustion temperature. Use the stock plugs for the 69 GTO heads. I have a personal preference for the Autolites.