Ever watch Graveyard Cars? Like the factory, they paint the engine assembled, manifolds, spark plugs and all. Only the porcelain on the plugs are masked more or less. The manifolds were pretty well painted all over. And if you watch Graveyard Cars when they fire them up, there is a lot of smoke and the paint for the most part disappears. It only goes black for a few minutes until it burns off. There is usually paint left on the nuts and the like, but most all the paint burns completely away on the manifolds themselves. So, I find it easier to just paint with the manifolds in place rather than go to the trouble of masking. And I use gaskets, but trim the excess as you suggested.