I think the width of the belt will make the belt want to track true. as long as the belt is true, and the axles of the pullies are perfectly parralel. I'm sure there is a arcane art to setting these things up. there is a tensioner on the steam tractor setup I observed the other weekend, and that is modeled in the Woodlands Scenic's rural saw mill kit that I may rob some parts from. Perhaps there is some adjustminets on the tensioner
I need to study the pieces of the Keystone saw mill carriage I have for the Berghausen-Shoemaker Mill in Crooked Creek. Keystone's bandsaw machinery are very very nice castings. I have most of what I need for the big mill, Perhaps building the machinery for the big mill would make more sence than scratchbuilding a third sawmill, which I don't know where I would put it yet.