Which 722.3 will work on a M103?
 

I'm faced w/ the feat of replacing the 722 transmission in my '89 300CE (m103).

There's a 722.311 out of a w126 500SEL available (w/ 2nd start) - will that work?

If not, why? Flywheen gear tooth differences? What other versions will work?

I can't answer your question but you should be aware if you aren't already that these trannies got a gear ratio change in 9/88 (on the later ones first is lower, second is taller) that doesn't effect interchangeability but does affect your ability (a little) to holeshot guys at the traffic lights if you are into that sort of thing.
Regards, Eric