There was a really comprehensive thread on this a while back, from memory any Borg Warner LSD centre (from a recent Commodore or Falcon) will fit. A VL turbo or Silhouette centre will probably be rooted by now. Something more recent would be good.
I looked into shortening an R31 diff, but some diff joints told me that you couldn't do it since the axle narrowed after the splines. Ring around a few places and check for yourself though. Also if you want a 4.1 ratio you can apparently use the pinion and crown wheels out of a Pintara no probs.