After hours of puzzling with proportions and tire sizes in excel and with the wise advice of various experienced sources, we have finally made the decision. We go for a pre-loaded Drexler bar and a pinion crown wheel ratio of 4.75:1
For the pinion and crown wheel we also chose Drexler. Given the fairly short ratio, we choose to build everything into a large 210 diff house instead of the original 188 house. We do get a few extra kilos of lubrication, but it certainly benefits the reliability. We removed the diff house from a 635CSI and as a lid we use this from an E34 M5 diff. For this it is slightly easier to make an extra bracket that is attached to the bottom plate.
The diff house will be sandblasted in November 2011 so that we can give it a hip color again.
After that, everything can be assembled and the driveline really starts to take shape.