I just did this with my 78 W116 300SD.
All the belts have to come off to install the alternator/water pump belts.

The FSM link is here:
https://www.startekinfo.com/StarTek/outside/12265/disc_2/program/engine_82_co.htm

Group 13 is the Belt drive section.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022