Using the ford 8.8" diff found in IRS mustangs has been used as an upgrade for high powered xr4ti's over in the states as it's a pretty common diff. It's not a straight swap need some brackets made up http://www.key-ideas.com/DGKurTwo2-040604-8.8LSD.html for mounting to the rear beam.

Another possible 9" option however probably even harder to get hold over in the UK, is to look into what some of the Aussie Group A rs500's used as they didn't fancy paying the cost of having to using Motorsport 9" diff and wanted to use something more local so they managed to get a Harrop Diff homologated for use, bugger knows what one but as they still like there big powered v8's over there you may still be able to get hold of something similar.