Schroby- I'm assuming you mean you blocked both rear tires up off the ground and tried to turn one. Gearbox in neutral or not, the other one should turn, the opposite way, due to the differential. If neither will turn, could it be the brakes are rusted engaged? If either wheel will turn 3-4 inches then stop, probably not the brakes. It doesn't seem likely they rusted in place, unless of course the guy left them locked on, then they easily could be rusted to the drums. If you can back off on the brake adjuster you might break them loose by trying to rock the wheels back and forth.
You've said "hydraulics engaged" but I'm not sure what that means - does this C have an internal pump? I only know As, that didn't. If such a pump ran directly off the transmission and it was broken/jammed, it wouldn't allow the output shaft to turn. Still, if the clutch disengages (are you sure?) the engine should turn (when you get it free) independent of the rest of the drive train. The clutch moves back + forth a little, but can you turn it at all with the pedal blocked down? You'll have to be careful of the friction surface but you might get it to turn a little with a slender block of wood? I never tried this. If the disc actually turns, with the tranny in neutral and rears blocked up, you only know the input side of the tranny isn't bunged up, something could still be broken back of that.
I don't know where you are, but could this machine have gotten water in the gearbox that froze sometime? That could crack a housing out of alignment and jam up gears and shafts. Have you checked for cracks in the castings? Have you checked gearbox and rear check plugs or drain plugs? See what comes out of the drains into a pail. Ice ain't good in there. These are very tough tractors, but sometimes enough is enough.
You can't lose much by pulling the head. Try tapping pistons with the block of wood almost as big as piston tops. Try the prybar on the ring gear idea. I doubt the hand crank will budge it, and I'd be afraid of putting too much torque on the crank damper bolt or nut.
If something in the drive train broke while this tractor was being used (plow in the ground, etc.) I could imagine the owner leaving it right there as is as was. Maybe thinking to get to it later, maybe giving up on it 'cause there were several things wrong with it. The engine could have rusted up later. Can you find anyone who knows the history?
Keep us informed as you go.
Jim