Got to go bigger as Mike says mate.

Even my brother managed to do a diff with a 7.5" Quaife, and wasn't sideways nearly as much as you!

Search about a little, and Jag diffs can be bought for next to nothing. I bought two Jag diffs when I was doing mine. One standard, and one rebuilt by TWR with a different ratio - think it was 3.77... Paid £250 for the pair of em! Obviously you'd need the shafts, beam modded etc, but I dare say that the setup would probably last.