I had the clutch on my 78 300D go out. When running the AC you could see the pump would not turn, and the pulley was turning and making a huge racket. I was able to confrim the pump was probably still working by removing the pulley and clutch set and using a wrench or vise grips to turn the pump. That will not confirm the pump is still intact, but it turned over freely so I assumed I had not blown the compressor.
Next was to locate another clutch assembly. Being I was interested in getting is fixed quickly and for as near to free as possible, I went to a local wrecker, with the clutch assembly in hand. Was able to match it up to another that was on an old Caddy. Cost me five. Easy fix. I think I had to use the old pulley. Also the bolts on the compressor on both the MB and Cad were not metric. Detroit iron all the way. Ran just fine with the used clutch.