As mentioned above the XTR mechs were designed for a 46/48 outer ring, the curve of the outer plate won't follow the curve of the ring on a 42, it'll probably work fine but looks odd in my opinion. Also, looking at your frame, the curve from the seat tube into the down tube looks like it will interfere with a low clamp, meaning it won't go low enough. I think I'd look for an XT M738 mech, matches the crank, designed for the right size rings and likely to be both easier to find and cheaper than an M953. I will admit to liking matching mechs though, so see your dilemma.
