Here is the washed diff just before installing it back to the gearbox. Not looking so pretty, but I think it should be strong enough. Last layer was welded with mig (with rather moderate power) and all the deeper layers with electrodes:
A couple of photos when I was assembling the gearbox back to one piece: I spotted a perfect location for a new oil drain plug since the original is so high at the gearbox that it leaves almost half of the oils still inside. This was a blank 10mm hole, which I drilled through and made 12mm threw to it.
And finally all bolts bolted back where they came from + all the parts inside the box. I tested all the gears and they seemed to go in and also looked to work as expected when I rotated the input shaft.Gearbox paired back again with the engine: