I'm afraid this upgrade is not viable. I asked the Leica technical engineer when I attended the Leica School up in Milton Keynes and was told that you'll need to change out the entire rangefinder unit and this unit does not fit the M6 or even the M7 exactly. The other thing is that at the moment, all the MP rangefinder units produced by Leica are to meet the MP bodies production quantity!
It's a real bummer as I would really have like the new coated prisms of the MP in my M7!