I just did this PITA 2 weeks ago on the W123 turbo , and spent way more time on it that it was worth. I made the mistake of trying to go in from the top (as I had successfully done on my W116 300SD a few years ago) but the bolt is even closer to the firewall in the W123. The bottom is really the only possible path forward. I used a 3/8" ratchet with about a foot of pipe on it and all the extendos and it just did cleanly break loose. I thought I had broken the wrench when it let go.

__________________
80 300SD (129k mi) 82 240D stick (193k mi)77 240D auto - stick to be (153k mi) 85 380SL (145k mi) 89 BMW 535i 82 Diesel Rabbit Pickup (374k mi) 91 Jetta IDI Diesel (155k mi) 81 VW Rabbit Convertible Diesel 70 Triumph Spitfire Mk III (63kmi)66 Triumph TR4a IRS (90k mi)67 Ford F-100 (??)