Nice trailing arms (in the picture) by the way what are they from?
{Oh and bear in mind the brake stay and LCA on the front of the W123 can be moved by about 5cm to may be 7 cm within the wheel arch of a W123 <= guess based on my experience of doing my own alignments}
{Oh and this thread might be of interest to you
Trying to fit an aluminium W126 / 107 trailing arm to a W123 - a question of compatib even though the aluminium arms are heavier than the steel}