I bought the belt up over the servo cells and motor and the lower belt side nearly touched the upper and the chasis was 7cm wide at the widest and the wheel base was 3mm shorter than the bj4 mark1
the tranny had the tensioner just infront /above the servo and never suffered belt slip even when it was fitted with sch yellow full spikes and a 6single with clutch locked up on long grass
the reason i built it was that i was challenged to build a slim car that had a very low cg,and balanced front to back and left to right.
so i chose the losi diffs because they are centraly driven and slimmer
the only trouble i encountered was it ran slightly hot and i think this was caused by a slight missallignment of the tranny case making the belt thrust slightly to the left, therefore tightening
could have probably sorted it but i just slung it in a cupboard and started something else
hopefully this might give you some ideas