I've found it necessary to reduce the width of the High Level gearbox, that comes as part of the kit. It's already quite a narrow gearbox, at about 7.5mm or so, but even that wouldn't fit between the brass bearings on the driven (rear) axle:
I first of all removed one of the gearbox sides and re-fitted it about 1mm further inwards, using some 10BA bolts and nuts to hold it in the right position for soldering:
When I was happy with the alignment, the side was soldered back onto the main gearbox and some additional strengthening pieces added:
The gearbox now fitted!
Now waiting installation in the chassis and test running etc.: