The 215mm does not just fit. I am retrofitting one to the w124. It is the same case as the C63 and others of that size/era. It will fit but requires custom rear cover and or custom rear subframe work. I tried about 3-4 different rear covers. Maybe there is one that works but I could not find it.
Unless you are planning on doing a wheelie the 210mm diff is more than up to the task. The axles are the week link and they start breaking around a 1.5ish 60' at the race track.
Any ways I have a thread on here about the setup I am doing.