Coolant options

Your engine makes X amount of heat, depending on compression, camshaft, etc. Nothing will make it run cooler. You can improve the heat rejection from the engine into the coolant by having a clean water jacket - clean and rust free. Some have filled their block with EvapoRust to clean it better than a flush. Newer thermostats are the biggest restriction. Look at 1967 thermostats like 2128994. Your radiator may be dirtier in the tubes than you think. Use distilled water and mix it 50/50 with the regular green coolant.