360 Tune Up

The thermostat sets the MINIMUM coolant temperature. It has nothing to do with the maximum unless it is restrictive or stuck closed.
The cooling system efficiency sets the high point. At 70 mph and in 5th gear, your cooling system should be at or near peak efficiency....... if it was working right.
It's not. That's why I'm trying to figure out what is limiting the cooling. The radiator is a Champion 3 row and is rated for WAY more HP than I have.