I just did this about 3 month ago on my 1992 400e.
You need an O2 sensor socket or 22mm open ended wrench. The sensor socket is the recommended way to go since you may strip the sensor if it is frozen in the threads with the wrench.
Believe it or not, there is a way to get the socket and ratchet in there. I lifted the DRIVER'S side and came in that way. I had better access coming in from this angle than when I tried with the pass. side lifted.
What I did was put the socket on the O2 sensor THEN I attached the ratchet to the socket. This way I had more room to move the ratchet around to get the best angle. Then I snapped on the ratchet to the socket.
It shouldn't be that hard. I think it took a total of 10 minutes for me to get the old one out and new one in.
Hope this helps.