I already tried getting the newer b4.1 diff in there. It is too wide and is not a direct fit. The original b4 diff might fit in the tranny case, but then you will have to get a 5/8 bearing in a 15mm hole. Before I start drilling out the tranny case for that I want to try getting an RB5 diff in there. It uses the same bearings at least.
For rear wheels, I've been running TLR 22 wheels using Jconcepts 12mm adapters for the B4, with one HPI 5x7x1.45 axle spacer and two 5x7x.2 shims between the hub and the hex.