This truck is definitely very nice. You do amazing work. Been a while since ive been on. Havent had much time for this hobby. I just sat down and went through this entire thread. Again amazing job!
Thanks for checking out the build and all the kind words and compliments. I have not been on the forum much either lately...

Sorry for the extreme delay in responding there BearFootfan.... just started making time and hobbying again myself...
Jay, I know this thread is older but definitely worth a bump

- what did you do to file down the steel idlers to fit? I want to do the same.
Thanks for the question and the bump.

I'll try and see if I can be of some help to ya...
The Traxxas part #3996X steel idler gears are 20T just like #3996 teflon gears that come standard in the 3906 E-Maxx transmission, but they are a bit too wide to install without them binding and rubbing on the inside of the transmission case. I used a dremel with a #426 1-1/4" Fiberglass Reinforced Cut-off Wheel and
very carefully cut/ground the face on both sides of each idler gear until it was nearly flush with the bearing. Afterwards, I cleaned up my cuts using a drift file and some 60 grit, then 100 grit sandpaper laid on a flat surface to get that machined look.
Hopefully this post was helpful.
Respectfully, - Arco