ok so i have just modified my 3906 tranny to use the 3908 slipper and deleted low gear and also locked the tranny in hi.
first thing i did was pull apart my 3906 tranny and this tranny(not sure what model its from but its from a t-maxx) the next is to take the input shaft out of both
as you can see the 3906 shaft is 15mm longer so i cut the 3906 shaft right at the eye
next i epoxy glued the cut piece to the back of the t maxx shaft to make it the same length as the 3906 shaft.
then i cut and ground down a hex key to fit through the tmaxx gear holder hole
i then used super glue to hold in the hex key and put everything to gather and checked fit
i had to use a good amount of shim's and a bearing to take up some slac on the slipper side
and for locking it in high i took out the top idler gear and put a clamp on the shift shaft inside the tranny