Get Specialites TA Chinook chain rings and short chainring bolts from SJS cycles , should be around £70 for the lot and bolt it to your existing XT cranks.
Make sure you buy a 40T middle chainring and mount it behind the spider with the short chainring bolts otherwise you chain line will be all to cock.
I have started using a similar set up and was using a 11-32 cassette, whilst 40/32 was fine on short dry technical climbs (Where shifting to the small ring would be more hassle than it is worth) it became more of a chore when it was wet so I have changed to a 11-34 which actually feels a lot better across all the gears.