Most time, overheating at speed is a flow problem. Get a thermostat with an 1 1/4" to 1 1/2" opening. You could try a good flush, but a scaled up radiator may not clean up without repeated flushing.
Wouldn’t you want the fluid to stay in the radiator slightly longer to allow it to dissipate heat? I’ve had a car with the thermostat removed and it actually caused a larger heat-soak issue because the coolant didn’t spend enough time in the radiator before it flew back into the engine.
