M30 E34 final drive options typically are:
3.46 - stock manual
3.64 - E32 M30 sources (if my memory is correct)
3.73 - E34 early 525 M20 sourced (if my memory is correct)
3.91 - E34 early version, M30 stock auto
4.10 - E34 late version, M30 stock auto
I've swapped in a 3.73 LSD from 3.46 and it's about as short as you want to go in my opinion. If I could come up with a good 3.64 LSD, I might step back to that ratio.
Bellevue WA
90 535iM - not much stock remains. 3.7 liters, ported head, cammed, 3.73 diffy, M5 brakes, MAFed, yadda yadda yadda
86 Porsche 951 - Track Toy