The differential is not a big deal.
The late model e24 uses the same model and size differential as the e30 6 cylinder, Z3, or the e28, with an output shaft swap and an aft face plate swap. The output shafts are a friction lock that can be poped with a screwdriver.
If yours is a gray market car it should be a 3.07 to 1 ratio.
Look for a tag on tha aft face plate on the differential.
The USA cars all have a higher ratio and that provides a bigger torque curve for better performance.
The ratio are 3.25 to 3.73 on the e24 635csi.
The ratio on the M series is 3.90 to 1 and it uses the larger chunk of the seven series, e23.
But you can get a 3.73 or a 3.90 to 1 from a e30 and I will promise a difference you will not believe.
I use a 205.60x15 tire also and mine uses a 21mm offset.
No rubbing anywhere or anytime.
Got the springs from Bavarian Autosport.
I used the Bilstein Sports and the second spring perch up on the rears.
That gave me an effective drop of 1.5 in the front and 1.7 in the rear. The tire size is very important in any suspension setting and mine are the Yokahama 205.60x15. I also have the complete Paul Gray synthetic bushings in the front.
Here is my close up picture and I have hand space all around and no rubbing ever. I have the tirerack hub centric spacers on these wheels since the hub size on the later models is larger. These are the BMW OE wheels and are polished and not chromed.
Later,