Managed to source a BMW E12 (5 SERIES) diff running the ratio we wanted of 3.9:1. The only problem is the fact this is an open diff. An original E12 LSD diff is very rare & if you can find one for sale they are very expensive.
I should mention the reason for using the E12 diff & not keeping the E21 diff.
The E21 diff had a huge problem of over heating back in the day when the E21 was raced. I've read quotes & scene pics of the diffs glowing red etc, this was because the diff casings were very compact which meant they couldn't hold much oil. Alpina did release a bigger rear panel which would allow the casing to hold more oil & it was heavily finned but over heating did still occur.
The E12 diff is relatively straight forward swap with only small modifications required to the rear subframe.
Now this diff will be fully stripped, all the internals checked, reused or replaced with new items. The casing & drive shaft flanges will be bead blasted & re-plated.
Hard to imagine looking at it now but this diff will look brand new once completed.
I have sourced a very well known UK company to make me a LSD & internals, their products are already used in high class motorsport.