I like to set the manifold on, then reach in and feel how it matches up. If the manifold is a bit smaller, like Pat sez, that's a good thing. Only if an edge is sticking out into the wind do you want to get out the Dremel. Doesn't happen very often, but worth checking. There's no advantage, and maybe even a loss, by having the port swell at the gasket and then choke back down.