I took off my stock exhaust and used the old piping to put in a cherry bomb. Similar to
this but I only did single out and didn't use any of the reducers. It has a great sound with no loss of flow/back pressure. Haven't posed pic's yet though because I'm still working on getting it to hang right.
I would stick with 2" piping going to the muffler, and if it is a higher flow muffler and you are doing dual out I would use 2" piping all around. Defiantly don't go bigger than stock on the piping.