Smaller pipes will reduce sound ~2" or less, but at the cost of back pressure and some power loss. 2.5" is probably preferred, 2.25" is good too with little power loss from 2.5". 3" is just too big. At least with 2.5", mandrel bent pipes probably aren't going to make a performance difference. Just cool to say you have them....
the resonators are awsome, they did the job of taking out the resonance and left some growl, still loud mind you but no longer embarrassing. Mine were just 2.5" dia. x 10" blanks. Look for glasspacks form your local muffler shop that just weld in, mine were small due to dual exhaust and tight piping, but if you do one exhaust, you may fit a bigger one that will do more.
good luck!