is the main power box connected to the ceiling adjustable to anywhere on the track? Or is it only to be mounted with the box centered?

only with the box centered

Mount in the center.

I wish I could be more helpful, but I installed the lights in another house, which we have recently sold, where I also left the paperwork. But being a guessing man, I would assume the transformer is in a fixed location which I don't see could be a problem.