yeah, i think you'll have an easier time with a mtb crankset. through my own research, i've heard that road triple cranksets may also have longer spindle lengths due to accounting for the third chainring, but this is all conjecture as i don't have a triple crank on hand to check.
just like i did on my ultegra crankset, i would try to get a good idea on what the tolerances are for spindle length. as we've come to realize with the phil wood bb, we get an extra bit of play due to the 9mm outboard cups. at least vs. ultegra/dura ace.
unfortunately, one bit of info you will want that you may not have is the outboard cup length of the mtb crankset you are wanting to use. i'm 80% confident that shimano xtr/xt/etc all have the same bearing size and, subsequently, cup sizes as their road offerings, but it's still a risk without having them on-hand to measure.
you could also extrapolate that spindle tolerance is 2mm (from what i have). that, plus making an assumption that xtr/xt bottom brackets also have 11.8mm outboard cups means (like ultegra/dura ace) 5.6mm of reduced length.
philw bb
xtr/xt bb
- 73 + (11.8*2) = 96.6mm
- this could be at the end of the tolerance (like my ultegra setup) and then a 2mm tolerance means 94.5-96.5mm install length for the spindle.
- so you could potentially have 1-2mm of play that needs to be set and fixed by a bb spacer
a lot of assumptions, but it may be okay to go ahead without performing any bb reduction.