360 Tune Up

Got any ideas?
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=~2450rpm in 5th gear, your cooling system should be at or near peak efficiency....... if it was working right.
At 70mph the cooling fan and shroud can be,or should be able to be, shut off or even removed. Ram air should be sufficient no matter what pump you are running and no matter what stat is in it, and no matter what coolant you are running. But this assumes the rad is of sufficient size and capacity and that the 70mph column of air the car is plowing, is all or mostly all passing thru the rad.