If the stump jumper is also the drive disk for the blades, I would take off the big center nut and go cut some brush in a field for a while until you hear the disk get loose and fall off the shaft. The tapered spline usually isn't stuck all that hard - just hard to get off with hand tools. The disk and blades won't get hurt falling to the ground. They'll be spinning but will quickly stop. You'll be up above it on the tractor seat out of harm's way.