Good question! I have many hydraulic gear box/motors that rotate the booms on bucket trucks. One idea was to put a gear motor on top at each side of the I beam. I would then take a 6" (ish) pipe and weld three flanges on (ends and middle) creating two spools on each pipe. I would mount a pillow block bear inline with the gear motor with the spool(s) in between. I would then fix cable in each section of the spool with one cable wound to go one way and the other cable wound to go the other way. Keep in mind that there would be a gear box/pillow block/spool/two cables on each side. I would then mount 4 turnbuckles to the headers on the building, one on each side of each spool. When you would run the gear motors one way it would be winding one cable (on each side of the gantry) while letting the other cables unwind, and vice versa for the other direction. I believe that by having a setup on each side would keep everything tracking correctly. If it doesn't track right, I have already bought some 1" x 2" x 1/8" channel that I would recess into the concrete floor with epoxy. From there I would have a bearing on each end of the A-frame on a stud that would protrude down into the channel. On a side note, where the turnbuckles mount to the wall, I would run a tube/channel/beam from the front to the back of the building so it wouldnt put all of the force on one wall, but distribute everything.
