FWIW, the problems with trying to fit a 6-speed box into a set of 5-speed cases, are:
The shafts that the shift forks slide on are smaller diameters (could machine up some bushes, IIRC);
The diameter of the gearbox input shaft is smaller (6303 vs 6304 bearing, if my sleep deprived brain serves me)(this can be solved by using the correct bearing and clutch);
The diameter of the shift drum is a different diameter (larger on the 6-speed?).
The last one is what stopped me doing it. I'd planned on fitting YZ125E internals into my DT125A cases, with 6th gear locked out (by brazing up the appropriate part of the shift drum), but it got to the point where things were beyond my limited machining skills and my sense of mechanical adventure.
FWIW, a 125C gearbox is totally different to a 125E box, but still has the same differences compared to the 5-speed.
Anyhow, cool project DJR.
In your position, I'd just race a 125C, but what you're doing is far more interesting.
