I think your W123 setup is similar to my W126 in the rear.
I didn't have to pull the trailing arm. With the rear tires on ramps, I used a floor jack to lift on the front part of the trailing arm, just enough to release the tension and unbolt the front bushings. Then lowering a bit to press the bushings out. I used some 5/16 all-thread, large washers and 3/4 Dr. sockets to press the bushings in/out.
The main thing is to use chocks, or similar, to lock the tire in place. This enables you to just raise the jack and rebolt. If you don't, and the wheel moves, make sure there are no sensitive ears around to hear the cursing. You will need to drop the entire subassemby to get it to bolt back.