I've tackled this in my own way and here is how I did it
335i 6AT with M3 DCT Diff and DCT Prop
Once you have the subframe mounted to the 335i the prop from M3 Diff to Center bearing matches up fine.
All that needs fabricating is the shortening of the M3 prop from Center bearing to gearbox.
I used the 335i tri bolt flange and rubber plate, had this cut off the 335i prop and welded this to the M3 Prop, this also shortened it in the process.
I took my measurements for the new prop from my original 335i prop, from center bearing to flange as this distance never changes on the car.
So in conclusion.
1, The center bearing location remains in the same position because we share the same chassis
2, The M3 diff sits more forwards on the rear subframe than the 335i diff
3, The Key is to always use the prop from the original Diff so this measurements is consistent
4, Use your flange from the 335i prop to mate to the M3 prop
Attached pictures for reference