Here is what I do. I leave header pipe attacthed. However since it i off. I completly loosen the clamp that holds muffler to header pipe. Spray lots of wd 40 into connection. I then attempt to twist muffler. In your case it might be easier to twist header(one or the other must be tightly secured). If it doesn't begin to twist free a little I then put something against the muffler at connection with header and with a hammer knock it away from header until it moves some. Another thing to try is put something under the muffler ends where the clamp is and pry up to loosen a little.  Use lots of wd 40.