You can fit the 4 pin diff into a disc braked axle, and also the shafts. When I did mine I recall I had to re-tap the bolt holes for the drive flanges (it was an early RR disc axle I fitted the stuff into)
That was what I got from Bill (long description of how it all works) at GBR.
The front disc rear drum seem to work effectively together.
GBR also has a fix for strengthing the Salisbury evidently if the oil is not changed they do wear.
Have yet to check the Salisbury on the 110 but it is on the list this summer. When I replace another set of drum brakes.