more than likely the old muffler at the gasket is rusted over, it will take some doing with some wd-40 or whatever you use ?, squirt in in around the gasket and flange, do some prying, work the muffler off the header pipe. sometimes they can be a pain to get off if they have never been removed in the past ?. but the muffler will come off !. just takes some twisting and pulling, a few curse words here and there, big hammer, lots of blood, sweat , and tears !..lol.