cycc, I was typing too fast and not putting things down properly. I meant heavy as in the touring bike is heavily loaded.
and as per bb width, for square taper Doug64 explained to me years ago the details of road bike vs mtb shell widths, bb axle length differnces etc and all those details of putting a mtb triple onto a road bike, but i had forgotten the details, but as you say, certainly needs to be researched properly if ever going that route square taper wise.
and yes, for a bike that will be ridden lightly loaded or unloaded a lot, a mtb triple is not the best I guess, but they certainly are great for loaded touring---but in this case, I guess the smaller granny option is just the most simple and will work well.