i don't mean to be all "i told you so" but i told you so. add a shim between the two gears. make sure that the larger pinion does not hit the larger spur when the clutch is fully engaged. remember, sometimes the larger gear has wobble so accomodate for this movement when shimming.
also, pre tap the holes that the screws go into the spur. use plastic safe threadlock to put the screws in. use an "X" formation when screwing the screws in to avoid wobble from distortion.
also use a long grub screw on the two speed shoes and take out the roller drum to avoid any issues related to the roller drum.
i found that sometimes a larger gap between the 2 speed shoe and the 2 speed housing actually avoids stripping. The stripping does not occur during shifting at WOT, it usually occurs if the shift point occurs too late and you hit the breaks at the same time. the two speed shoes grab the casing, whilst it is still shifting down and causes the stripping sometimes.
use a bigger gap if you are ona small track with a lot of quick up and down shifts and braking points.
do the above, eliminate the gear wobble, space the gears out and don't let it shift too late or shift too close to a corner entry point.
