The 123 chassis doesn't have the frame problem that the 114/115 chassis' did.
I suspect the problem is the coupling between the box and the steering shaft (between box and steering wheel).
BTW I have fixed many of the frame problems on 114/115 cars and placing a plate on the outside is not the answer. The frame in that area is a rectangualar tube about 6-8 inches tall and 3-4 inches thick. The heavy steering box is bolted to one side by bolts that tighten from the other side. If this was the whole story, tightening would crush the frame as it isn't much ticker than the fender in material.
Inside the frame at the attachment point are three heavy tubes that the bolts pass through. they space the side of the frame apart and when tightening they are squeezed. These tubes aren't loosely installed into the frame. They were originally built into a box that was inserted at manufacture and the whole thing spot welded together.
Once the frame has failed the first time all the internal box and tube structure is broken. For this ever to work securely again a good welder must cut a 3-4 inch diameter whole in the outside frame rail. Then he must reweld through the hole all the internal tubes. We also add some new bracing internally between the tubes. Then the hole must be rewelded and all cracks welded and reinforced.
We didn't come to this repair quickly. It took redoing a number of frames and a lot of exploratory surgury to come up with a repair that lasts.
Bosch Master, ASE Master, L1
33 years MB technician