The mismatch on the exhaust won't really make much difference, the port is plenty
wide enough for satisfactory flow, and the engine isn't a high RPM screamer anyway.

There are other high temperature sealers on the market, but the black Permatex is
the best for really high temps, so why not use the best. It is specified for applications
on exhaust manifolds that do not use gaskets.

FBD.