A sometimes useful trick for loosening the adjustable side of the fan sheave is to find a short piece of steel or even a block of wood that can be jammed between some part of the tractor or engine that won't move and the ribs of the back of the sheave to stop it from turning with the rest of the fan hub.

Remove the locks and spray lube/wire brush the threads on the hub and then jam it with your bar and use the fan blades to unscrew it. I usually resort to some light tapping as well. (the rear half of the sheave is the "nut" part that moves back and opens up the groove)