If it fails, its not the end of the world. You just get it reinspected. They're not going to be jerks about it, they'll even give you a couple weeks to do it. If you can get a receipt for a different muffler thats all you need. But if you take it and it passes, then your better off. It's worth it to take your chances.

^^I agree with that.

you get 45 days and they WILL check that the muffler was changed. you don't need a receipt if you did the work yourself.