intake gasket ?

So the thread wasn't about the exhaust gas passage but about the atmospheric relief in the casting that allows absorbed heat to escape. One can effect the other though.
If you have a working heat riser in the right exhaust manifold, bottled up exhaust would probably get hotter than flowing exhaust at right head, until the heat riser opens anyway. Right head hotter than left for a time? Of course if you run free flowing headers, no heat riser, there probably wouldn't be a lot of exhaust gas pushed upward anyway. Even better if you have a X crossover downstream in the exhaust ( levels the back pressures L&R ).
Back to with working OEM heat riser, block the crossover passage bottles the heat up. Fill or otherwise close the vent that lets the heat dissipate from the casting... I wouldn't.
I did see a copper mesh material poked in these vents once. That owner cut up a pot scrubber? Stopped his small parts loss anyway. Hand form polished stainless screens for "looks"?