Safe to shorten a dual-pivot caliper mounting nut?
To shorten a bolt, get two nuts with the matching thread.  Screw them onto the bolt and "lock" them together (tightened against each other) such that the nut surface farthest from the head lines up with the place where you want to cut.

Place the bolt in a vice (taking care to not mash the threads) and use a fine-toothed hacksaw or other cutting device to cut the bolt off flush with the nut surface.

Then unscrew the nuts and dress up the bolt threads with a fine triangular file.

If instead you want to thin out a nut, again fasten two nuts to a matching bolt and lock together.  Have the nut face you want to cut away facing "out", and have the nut screwed onto the bolt to the point that the bolt end is nearly flush with that face of the nut.

Use some sort of grinder to grind away the desired amount of the nut.  Then "unlock" the two nuts and screw them farther onto the bolt (to clear the threads), then off.  Work a fresh bolt through the nut several times from both directions to assure the threads are clear.