header wrap on stock exhaust manifolds?

For cast iron, put it in a box and send to Jet-Hot or somebody who does a similar (mil-spec) coating. You'll notice a slight reduction in temps, but not much. Remember that the radiator is upwind. Noise wise, cast iron is as quiet as it gets already. Make sure your gaskets seal completely and install an underhood pad. Header wrap is a major pain in the a$$ and should be avoided on any rig you ever plan to drive in the rain.