I think I know what caused the bearing to fail - loss of pre-load caused by the collapsible spacer holding it in place being deformed.
The out put shaft has a large taper roller bearing sat behind the bevel gear, a really good interference fit on the shaft, which at this location diameter is slightly bigger than the rest of the shaft - it means the bearing clears the rest of the shaft and only has to be pressed onto it's location, not the full length of the shaft.
Next comes the collapsible spacer, then the bearing behind the output shaft (obviously the shaft is inserted in the case before the last bearing). The spacer sits on it's own location diameters, the final bearing inner race butts up against the spacer. The other end of the spacer butts up against the shoulder of the first bearing location diameter on the shaft, rather than against the first bearing inner race. And over the course of around 90,000 miles, the loading on it has hammered it slightly over the shoulder - losing the bearing pre-load.
I've reassembled it with some stainless shims to spread the force a bit, and to allow me to crush the spacer a little more on setting the rolling torque of the shaft. A similar arrangement in the Lancia integrale box uses a rolling torque of around 1Nm, further reading around backed up this typical figure, so that's what I used, plus a bit for the drag of the oil seal on final tightening.
It's taken me a while to get it all back together, as I decided to change the timing chains while the engine was out, blast, paint and underseal the sub-frame, blast and powder coat various brackets, battery tray underside brackets etc, remake the bumper.under tray mounts that bolt to the lower radiator mounts and sort out sheared screws etc.
I bought the timing chains, guides, tensioners, gasketc, from GM Parts Direct in USA - slightly delay due to them not sending one of the guides, but all sorted and changed easy enough.
I then spent a week in Bogota (some Alfas floating around over there) and so only managed the first test run yesterday - 34 miles of silence - no timing chain rattle, no bearing hum :-)
Bit early to say its done and dusted, but fingers crossed . . .