Two ways to go either sleeve it on the inside with a tube that has the same OD as the ID of the moffler/pipe, or pick up a coupler with an ID that matches the OD of the pipe/muffler.
Unless you have a bracket on the muffler you sould have at least two inches on each side of the 'seem' otherwise it will easily work itself out.
What I've done on hard to fit mufflers, odd size pipes etc for which they don't make off the shelf parts (my old mustang headers used an oval collector for one) is to pick up some light stainless sheet, cut a strip between two and four inches wide then wrap the joint a couple times and use two stainless hose clamps on it. position the clamps so all you see is the strap and it looks very good. also requires support of the muffler, or a very light muffler.
ken.
