Most shims are 34.9 to 31.8 or 34.9 to 28.6, think you will struggle to find one that is 31.8 to 28.6, although I think there was an XTR m960 shim of that size.
Also, in case you didn't already know XTR m950 is designed for 8 speed.
Should be getting a Kilauea frame later this week & will be similar build but with rigid forks & XT m770 groupset