On my '72 250 (W114) I used a 1/4" thick by 6" wide by 8" long, steel plate on the OUTSIDE of the frame rail.
I used 3 galvanized pipe sections, cut precisely to length, as spacers, inside the 3 original bolt holes to transfer the rotational force from the steering box directly to the steel plate. I removed a 1" x 2" section of the broken outer frame wall near the bottom steering box bolt so I could install two 1/8 thick by 1" wide by 4" long steel channels holding 4 metric nuts on the inside of the outer frame wall. The 1/4" steel plate extends down and anchors to the undamaged lower part of the frame rail where I added the channels and nuts. I had to drop the steering box to access the broken bolt stub which was seized into the box. Instead of 2 long and 1 short 10MM bolts, I used 3 bolts, grade 10.9, in the steering box. These bolts are slightly longer than the 2 long original bolts because of the added thickness of the 1/4" steel plate. It's been working fine since last Fall with no more flexing of the steering box or broken bolts.
Someone on the Vintage Mercedes forum said that Mercedes used to have a repair plate/kit availible for some early '70s models - 108 or 107 cars, I think. I don't know if it would fit the 114-115 cars or if it's still availible.
Happy Motoring, Mark