Don't rule out air backing up under the hood at high speeds just yet.
One way to make sure/rule out is to install mbar manometers in front and rear of radiator support and run lines into the cabin to view them.

Unlikely, but headgasket issues would also show up after shutting the engine off and restarting it shortly after when coolant pressure is injecting coolant into a cylinder. But it's more likely engine combustion pressure is making its way into the cooling system when the engine is running hard.
Hard radiator hoses would be a result of that until pressure drops again.

Where's your timing at at higher speeds?
Fast/Slow curve?
Vacuum advance hooked up?
Late timing will cause warmer running engine.