Join Date: Sep 2000
Location: Washington, Michigan USA
The thermostat won't make it run any cooler - it just regulates coolant flow; it will only run cooler with a cooler-rated thermostat if there's enough existing extra cooling capacity to reject more heat than it did with the original thermostat. From your description, your cooling capacity is deteriorating (probably due to the gradual buildup of scale and corrosion in your radiator, assuming the rest of the factory cooling system is still intact - seals, shroud, fan, clutch, lower hose not collapsed, etc.). If all of these are in place and working properly, chances are you need your radiator replaced or re-cored to restore its original heat rejection capability.
'69 Z28 Fathom Green