If you can find them, change out the FLAT BOTTOM steel casters to a high strength polymer wheeled caster with a curvature to the wheel at the floor contact area. These will turn much easier when positioning the gantry. Same applies when looking for car dollies that go under the car tires (four of them of course) so 1 person can roll and position a 4000 lbs. vehicle by themselves.
Years ago I bought some of those cheap steel wheeled (flat bottom) caster car dollies like HF sells.......when I got them under the car, 2 guys couldn't move the car around! I regulated those 4 dollies to engine dollies only! I recently bought the HF Daytona car dollies and they work fine for lighter cars and trucks......I have them under the 1979 VW Bug currently, and they are perfect for that. If they would make some aluminum plate dollies (these are steel plate) they would be much better.......the steel ones weigh a ton!