I hate the seats in these cars too. I rebuilt the seat in my car, and while it was worth it, it is nothing like the seat out of a modern car (i.e., still soft and unsupportive). The corbeaus are a good idea, and the company even sells a frame so you can install it in the w123, but the problem lies in how to connect the seat belt buckle to the new frame and be sure its safe.

Don't know if you can install a seat out of a w124 or w126. Question has been asked many times, but apparently no one has ever tried it. I know the seat in my w124 is much lower than the seat in my w123, so I think if you were able to install it, the position would be all wrong.

Greg
'84 300D, 176k
'90 300CE, 165k