Just adding the header and increasing the Exhaust to a 2 1/2" with a free flow muffler will give you the best performance.Those aftermarket mufflers that everyone seems to enjoy hearing will cause you to loose a little bit of power because they use back pressure to get the trawny sound.And realistically speaking you are only talking about gaining at most 25HP by adding the Header and good Exhaust.Any back pressure in the exhaust system at all defeats the purpose of a larger exhaust and headers.