As you can see the wheel spacers i make and the gear boxes installed. I make a plate to mount the gear boxes on then install as a complete unit. The hull sides are slotted to accomplish this. They can be adjusted this way to tension the tracks without a separate idler tension-er.
i can give better details if you need them. These where all done to use the Tamiya Leo 2A6 tracks and I also used the Tamiya Leo sprockets. i just turned them around and drilled and tapped a hole to accept a grub screw to fix them to the axles.