That header is mighty short now. As you mentioned, hardly can be cut back any further.
It be interesting to try out a few different pipes and compare engine performance. I believe that Kato drew and used Hatori's original 200 series pipe which is pretty small and looks better suited to a 10-18 size engine. The 300's are about the size of a Mac 3.5 quiet pipe if memory serves. Pete kindly helped in obtaining both 200 and 300 pipes so I figured I'd eventually test the engine with each.
The muffled Mac's certainly fits with ample space though now that it's tuned. Will you be able to route the exhaust behind the TE former? Maybe a stinger extension?
Seems like paint and covering is all that's left to do! Maiden this Fall or next Spring? I guess you guys have better weather than we do up here...