If all else fails, determine the size of the bolt (i.e. 2.5 mm, 3mm, etc ) and buy a length of fully threaded rod and some nuts. Double nut one end and solder the nuts and rod together. Cut for the length you need and then use a dremel and cut a slot IN THE TOP BOLT ONLY. You now have muffler bolts the length you need.

I did this in order to fit some muffler extensions to my engine. They work absolutely fine. Cost was about $6 for a 3' threaded rod and another $1.50 for a small plastic pack of nuts. $7.50 for a lifetime supply of muffler bolts in any length you need.