The 3903/5/8 have the strongest stock Maxx diffs. IMO aluminum inner diff cups aren't needed, but adding aluminum outer diff cases will increase the strength a bunch. The plastic outer diff cases flex messing with the mesh stripping the pinion and ring gears. Shimming the ring and pinion with metal shims instead of the stock Teflon seals will help also.

The FLM hybrid bulks are a great solution. Not exactly cheap by the time your done but cheaper than the UE and aluminum bulks. I used them on a build with Ofna Hyper 8 gears with no issues on 6s with a CC1520 motor. I'd get the combo kit with the diffs included as the diff out drives will be setup to accept the stock plastic sliders or any cvd designed for the stock diffs.