John,
I'm no aerodynamics major, but if you are going to make a new pipe and muffler, I would put the muffler up closer to the header down pipe and have a longer exit pipe. This would give the exhaust gases more time to settle back down into a nice laminar flow before they exit the tip, giving them more speed and less burbling. I also would not flare the tip. When someone on LOCOSTUSA did an airstream model on the Seven bodywork, I was amazed at how much of the air that hits the rear fenders flows up into the cab rather than off to the side.
Tom