Remove some of the outer pans to reduce the width of the disc. That's what I did to my 13-ft tandem wheel disc so my Mahindra 5525 can handle it.
Some folks cut down the frame and the axles on an oversize disc. I decided not to do this. I don't need to work that close to fencelines and the extra weight of the full-size frame is beneficial.
/ What to do if your tractor is too small for your disc harrow...