With all due respect, I think that you are making a mountain out of a mole hill. These sound like normal underhood temperatures, particularly after a long drive at highway speeds. The components under the hood are designed to withstand that level of heat.

I am also curious as to why you are replacing the exhaust manifold. Mine is pitted and discolored, but does its job just fine.

Swapping in an M46 will definitely improve fuel economy and cut down on engine noise, but I doubt it will make an appreciable difference with respect to engine temp.

Heat wrap can advance heat related metal fatigue. Not sure if that applies to cast iron... But that kept me from wrapping my steel pipe header. For sure do not wrap the steel down pipe.

Like Michael said, ceramic coating would be better. Expensive and require sandblasting type prep of the manifold.

Also, wrapping/coating the exhaust may change the pipe temps and change exhaust function... Which might be good and is exactly why you see motorcycle exhaust with heat wrap tape.

I had a heat related problem with my aftermarket header burning up alternator voltage regulators. I relocated the regulator with Dave Barton's adjustable unit.

An electric fan conversion may cool the motor better at lower speeds. ??? That would be up for debate.

First off - take some actual temperature measurements before going crazy -- no reason it should be any warmer underhood than anything else.

Ceramic coating the manifold/head pipe inside/out is your best solution. Also, most expensive.

Heat shield will keep the exhaust manifold from radiating excessive heat into surrounding components, but will do virtually nothing for underhood temps. The shield blocks radiant -- but the vast majority of the heat is still present under the hood. Wrap and ceramic coating will keep more of the heat in the exhaust stream. Higher temp exhaust gas AND - the downstream part of the exhaust system will get hotter. And it's close to stuff too.

So - no Santa Claus....