If the muffler is difficult to get off, chances are the nuts have rusted onto the studs, or less likely the studs have rusted into the cast iron cylinder casting. Applying a big spanner will probably break off the studs rather than remove them. Extracting broken-off studs can be difficult for amateurs. I suggest you use some science: wet the exhaust studs and nuts with Penetrene repeatedly for a couple of days, then when you are ready to remove them, heat the nuts with a propane torch before you try to unscrew them.
