How do you stop a 68 Pontiac Fire Bird from overheating?

flush out cooling system, install a new 160 degree thermostat, make sure radiator isn't plugged up, install a radiator shroud with a 6 bladed fan blade, you can also add a product called water wetter to the radiator and it will lower the temperature by 10 degrees, summit racing sells it.

There are also two seals behind the metal plate behind the water pump that have to be replaced. As they get old they harden, crack and cause an internal leak in the engine so that the water circulates only at the front of the block preventing the water from travelling through the entire block. The sleeve assembly part number from GM was 9796347