The MTW muffler is a good one, and gives more power than JMB. At the same time the engine runs cooler.
For 3D, it is better to have the header slightly longer than for best rpm on your prop. The MTW likes to have the header adjusted well. A slightly longer header will give very crisp throttle response.
The same goes for tuned pipes. Tune for midrange response, and not for best power. In other words, first find best power, then go back a step or two to the longer header length.