So set the lowest aperture. Set the lowest shutter speed you can shoot with and keep the ISO between say 100 and 400 or even 800. I do not have an M so I can't confirm the exact settings.
I support all but "set the lowest aperture". When using an extender, even the fantastic 1.4xIII, I found it better to close down the aperture by typically 2/3 to one full stop to get maximum sharpness from of the system. This holds also for superteles and even more for the 100-400. But as always: improve the weakest part of the chain...