Yes, they are interchangable; but why go through all that trouble when you can buy a new, improved-design box spring? It would involve less work (only requiring you to remove one seat), and the end result would be much more satisfying. The passenger seat is also probably beat after 20 years. I replaced the box spring and seat pad on my old 300D and it was definitely worth it.

I think the seats are the curse of the w123's (and w124's for that matter). Its the one thing about my old 300D that I DON'T miss. Saab and Volvo were making awsome seats in the 80's, so I don't know what Mercedes was thinking.

Greg