Ideally, a 1.4X TC would be my choice. Unfortunately, the best TC that physically fits (Kenko 1.4X DGX is known to lockup the 5D MK III. I was able to confirm this the hard way)
I was able to use it with AF and a shorter extension ring. I handhold the lens, so I had to hold my breath and be very still.
5D MK II, 100mmL f/10 ISO 800 and a substantial crop.