Shaping up really nicely man! I am torn with the muffler. One thing for sure will be an issue is ground clearance, as doc already mentioned, but if it swept up slightly this would eliminate than. Then the other thing is the style. You are blending (very well might I add) old with new, so it just depends how much of each you want to through in the mix. This can looks unmistakably modern - suits the engine and front end well. However the frame, seat and tank lone themselves more to the classic style and a reverse mega would look best here... Perhaps one of those nice SS ones from Purpose Build Moto or Cone Engineering or even an M4 reverse mega 8)