Simply take the muffler with you to the local hobby (hopefully it's local), that makes it a hole lot easier for the salesman to help, and advise you. Any questions may have will show up at that time, and he can clear them up. This is one of the benifits of having a good working relationship, with your LHS.
You will see how easy it really is once you have done it ,Have fun fly LOTS, Snuts.