I don't know that it will in his case as I think there is something else going on within the cooling system. Might be timing but I would be more inclined to think a hose that's failing or a block full of rust. .How is a higher temperature thermostat going to make it run COOLER on the highway?
Since the only thing that has not been replaced on mine in the last 3 years is the heater core or a block flush I'm guessing.
Mine stays at a consistent temp slow or fast with a stock 4 blade fan& stock size radiator. I don't even know what the manual calls for in a stat only what got me where I needed to be with the Sniper
Then maybe mine disipates more heat running a split header over the cast iron restricted manifold,with the alumn carb heater sitting over the combined #3 & #4 exhaust port.
Again just a thought.
But if mine will run up the interstate 70 /80 mph with a 190 stat & not overheat his should also. My 66 with a stock everything 200 6 did years ago.