Iam sure the 123`s use the same length rear section. Iam not sure with the w123 300CD/280CE, that rear section may be different.
the 300TD and 240D/300D rear section is the same length. the diameters will be larger on some. I have a 240D the one off my 85 300D, and one I installed off a 80 300D. that last one has a larger diameter than the other 2. the 240 being the smaller diameter.
I used the rear driveline from the 80 300D, it only had 160,000k miles on it compared to my 34+,xxxk miles. that was like new compared to my high milage one.
This is a good time to replace the center bearing now you have it apart.
when I had the front section from the 240 shortened, the shop charged $150.00. they also will replace the MB U-Joint with a Nissan sealed none greaseable joint. some driveline shops replace them with one with a zirk fitting. if I have it done, I`ll go with the Nissan joint. so far for now, it is running smooth as can be.
when you finally get the transmission installed and things all connected back up, you will have to eliminate some of the throttle linkage. like the Bowden cable and a few things, don`t remember all.
I just replaced the plate and two rods on the valve cover with one off a 78 W116 300SD. I drove it today for the first time, and wow what a difference. the peddle is smoother to push down, and the jerkyness and driveline vibe`s etc are mostly gone.
one of the two rods has a threaded section, and sort of a slip joint, think that is what really smoothed it out mostly.
www.peachparts.com/shopforum/showthread.php?t=266013
Charlie