AC WD I think

Soak,soak,soak,Rock,rock rock.Hand clutch must be pulled back(enguaged)and trans in 4th for best results.You'll be ahead to pull each piston out and clean all the crap off/out of the rings ,pistons and cyl walls. If there's a lot of ridge at the top you could have trouble getting rings up past it with out cutting it out with a ridge cutter.Some careful work with a die grinder can get the job done instead of ridge cutter.Just need to "break" the corner of the ridge and do a lot of wiggling without too much force to get the rings past ridge.Rod caps have shims for setting clearance(unless someone has replaced with precision ground ones) so pay attention to what's where and keep everything seperate and matched.Also keep them in order(1,2,3,4) and scratch a mark on top pointing forward.They are usually marked in some way but just to be sure.The rods are not centered in the pistons so they need to go back in the hole they came out of.If rings are still stuck soak in a can/bucket till free if wanting to keep/re-use 'cause they break easily when prying/digging around. There will be plenty who say "new parts" but if you clean everything up well and put it back to gether,it will be as good as it was before it got stuck. Squirt some oil on the valve stems and tap with a hammer to make sure they are all free.

Nautyboss, it sounds like you have both rear wheels jacked up.  If so, even with both clutches engaged, rear wheels will turn in opposite directions due to the nature of the differential.

Since you drove the tractor to its current location, it is unlikely that the tranny locked up or jumped into two gears just sitting there.  If you get the tractor running, you may find that you don't have a tranny problem, other than the heavy oil has probably turned to mud from decades of neglect.

If you PM me for more info you will see that there are interlocks on the forks so that you can't get into two gears very easily.  However, after 60+ years, the shifter tip wears out and other things wear down or gum up.  I have a '51 WD and have spent too many hours with my face over the tranny & wet clutch and under the PTO drive, but even I could straighten the problems out eventually.

Since there's been lots of good suggestions, I figured I'd post my method of breaking free the worst patients I've ever dealt with.

1) remove the head, vacuum out any loose crud.  I get a nylon brush impregnated with light abrasive, and use it to scrub the worst of the crud from the upper bore, then vacuum it out some more.  I clean the valves and lap 'em so that I know I'll get a good seal.  I clean the head gasket surfaces, grease and install, but leave the valves closed (on a flathead, it means removing the tappets or pushrods).

2) Remove and smash the spark-plugs... discard all but the thread-in portion.  I weld a piece of 1/4" steel pipe to each one, then thread 'em into the cylinder.

3) Mix up a 3-way of diesel fuel, hydraulic fluid, and acetone, and fill every cylinder to the pipe.

4) Make a splitter-hose (manifold), connecting all cylinders to a regulator valve, connect to an air-compressor.  Set the regulator to about 30psi, and walk away for a day or so.

5) Disconnect hoses, pull dipstick, and sniff it... smell acetone?  It's workin' through.  None? Refill cylinders (keep notes on which ones need more), rock the crankshaft a little (don't force it, just give it a little try) repeat 4 & 5.

Eventually, you'll start smelling acetone, which means it's working through from the combustion chambers to the sump.  The cylinders that pass first will USUALLY break free first, so when I have one flowing good, I'll disconnect that air hose, and concentrate on the rest.  Once they're all flowing, I'll hook 'em back up and dial pressure down to 10psi or so. 

I won't say I've never had an engine stuck so bad that this wouldn't break it loose, but it's been the most successful method I've used.  It isn't a quick job, and when you're working on an engine like an Allis W-series, it's faster to pop the head and pan, loosen the rods, and push the rod/piston/sleeve out as a unit, and find replacements... but if it's a cast-en-bloc linerless engine, particularly a V-type where rods won't clear the crank without turning, it's the only answer short of drilling and smashing the pistons to pieces.