It depends on the engine and who's seal your using. some of the manufacturers exhaust outlets have some diameter variation depending on which engine it is and so do the gasket makers. unfortunatly this makes the header fit somewhat of a compromise as we could never make enough combos to cover them all. It just would not be realistic. The GS seals are the worst for fit. Nova black ones are pretty good, as is the origional on most nova engines.
This is a way to fix the problem, It is what I do when the seal seems a bit loose. get yourself some medical tape at the pharmacy. the same stuff you use to tape a gauze bandage on with.(the white kind with fiber cord in it) cut a small strip the same width as the groove in the exhaust outlet stump of the engine and wind a couple wraps of tape in the groove before you put the seal on the engine. It does not burn and stays put under the seal until you take it off. it also helps to keep the seal in place better. somone else turned me on to this a few years ago and it really works well.
Dennis