I used to have the Major and did change the belt but cannot think that far back as to what I had to do to change it.
So is there any chance of a photo or three to give us a look at where you are in the headstock and the screw that is causing the issue.
Two things do immediately spring to mind, is it a left hand thread, does it undo in the wrong direction. Or has it been put in with a threadlock, that is normally released with heat, so do you have a way to direct heat at the screw, a small kitchen or plumbers blowtorch or even a good electric heat paint stripper.
