I came across an R31 Silhouette being wrecked and grabbed the diff from it (see: here). The ring and pinion gears are stuffed (pinion especially), its a 3.7 ratio (which is not what i'd want to use).

As i understand these are 78 series Borg Warner models. As are the live axle Commodore diffs. Can the 28 spline axles and centre from a Commodore diff be used in these housings? Or if i am getting it shortened would i be better off getting a Commodore diff and getting 4 stud hubs fit onto those axles?

Or should i just get a new 3.9 centre for this diff (would that just involve getting new ring and pinion?) and get this one shortened?

Are all R31's solid axle there? Here it's just the wagons.

My parts fiche shows a solid axle with H190 in the japanese poverty pack model. ( for Taxi's ????)

Or will i have to bite the bullet and start pertitioning diff shops for quotes/info on what i want?

G'day,
In my car I have a shortened R31 pintara diff which after seeing your's looks very similar if not the same? You could possibly use parts from these and if you could you can get these diffs in 3.9 and 4.11. Don't know if this is any help to you.
cheers

yep, the Aussie R31 pintara and skyline diffs are the same. Common mod for R31 Skyline owners is to swap in a 4.1 from the Pintara.

ratios
Pintara Auto 4.1, manual 3.9
Skyline Auto 3.9, manual 3.7
(Silhoette, Ti and GTS had the LSD)