Blocking coolant crossover?

If you don't need the heat for driveability in cold weather.... but it seems like it would be good for any street engine in Ontario to have the crossover. Is the exhaust manifold stock and does it have any heat flapper? If not, then it will be likely too hot in summer with the crossover open to the exhaust all the time.

If you cut the hole, I'd use high heat RTV (red) around that hole.