Hello, in regards to changing belts on the W123 chassis, there are a few differences between model years. For example, the early model years (1977 and 1978) use a different A/C compressor and the method for replacing that belt is different than on the later models. So it would be helpful to know which year you have.
However, in the meantime here are some links that may be helpful. They are PDF files from the online factory service manual. The first is for the early style belt setup with the older compressor, the second is for the newer style. If you read the documents they help you identify which you have.
http://www.startekinfo.com/StarTek/outside/12265/disc_2/program/Engine/615/13-340.pdf
http://www.startekinfo.com/StarTek/outside/12265/disc_2/program/Engine/617/13-340.pdf
I will probably do a guide on this with full picture instructions next time I do something like replacing my water pump on one of my W123's. Look for that.