Another thing to do is go to a muffler shop, and have them "swell" the pipe out with their equipment. It takes a few seconds, no adaptors, less pieces to have to deal with, and looks better.
Most places around here don't even charge for that.
But on the down side if you ever went back to stock, you would either have to cut the pipe, or just not re-use the crush gasket. Most muffler setups especially on Harleys, don't use a gasket.
When I get my system finished, I am going to use some anti-seize between the pipes so they won't rust together as there won't be any gaskets between them.
Just a thought.