The 114 through to W126 all use the same design rear subframe. I wouldn't bother with fitting a W124 multilink rear assembly in there. What you need to find is a diff assembly with anti squat links, You can recognise these because they have alloy lower arms with 107 part numbers.
All you the same rear brakes but the anti squat rear ends have different brake hoses.
Doing this,you could bolt in a W116 450SEL 6.9 diff with LSD and anti squat with a high diff ratio for lower fuel consumption ( I know someone who has installed one in a W123 280CE ) or a W126 560 diff to add ABS .