The 580EX can be set manually, so you won't have any problems. It is a bit of a waste of the capabilities of the M6TTL and the 580EX, but it will still work. You'll have to do the calculations in your head since the flash itself doesn't have a distance/aperture scale. You can calculate the power to use based on the guide number of the flash, film speed, distance, and aperture. The calculations will only really be accurate if you're aiming the flash head straight at the subject.
Whoops just realized I brought back a very old thread.