I needed a longer center rear dogbone, the UE "DogBone for Monoblock with the Thunder or Lightning Chassis" fits but it's a tad shorter IMO. The O'Donnell dogbone is longer but I use the UE one which seems stronger. I will test both though if needed.
I replaced the FLM drive cups with the Traxxas Drive Cups Inner Revo (5153R) which are longer. And I use HPI rubber O-rings to secure the dogbones.
Second step was the assembly of the UE diffs. As I use the stock driveshafts (I will use the Summit ones if the stock ones don't hold out 6s), I needed to replace the UE output drive cups (on top of the pic) by Traxxas stub axles (I cut the threaded part) :
I use Ofna diff oil (7k front & 5k rear), Acer Racing bearings and Gorilla Snawt (for ring and pinion gears). And voilà ! :
It was really long to shim them, I hope I did it correctly