I don't think it's a good idea to completely block the front axle movement. My International has a LOT of axle movement, by design, clearly, but it's a lot more than the Yanmars I own have. Were I working in some of the terrain I've seen, I would back the stops out a little bit, to more like what my Yanmars have. I'll make up a number and guess 15 degrees, rather than the 25 or so it seems mine will achieve.
If you have no articulation in the axle, everything you hit will be tipping your machine over. If the rear of the tractor is on some obstacle, the hinged front axle will let the machine flop; my International will flop further than the operator could stay in the seat. I don't think that's true of my Yanmars.
A major problem I see is the idea of "finding the limits" of the machine. Statically, the tractor will tip much farther than any operator should be comfortable sitting. When moving, the forces are so incalculable that I don't see how an operator could find the limits without reaching them. Reaching the limit on a slow steering, no reserve acceleration or power tractor is going to mean a rollover.
The cheapest thing to do is move the rear hubs out as far as they will go. Set up your rear wheels so that they are spaced to maximum. This will also help the most. Then, change what you're doing that makes things risky. As was suggested, don't skid logs with the loader. I've seen people who have built small logging arches for the three point hitch on little tractors. This seems to give them much more capability, safer operating conditions, and less abuse on the tractor.
Then, fill your tires with your choice of liquid ballast. If you don't like calcium (I don't like the idea either, and don't use it), try windshield washer fluid or beet juice, sometimes sold under the brand name "Rimguard." If you can, buy or make cast wheel weights. If you can't safely do your job with those three changes, and still have a reasonable safety margin, then you'll have to do something to change the operating conditions. That may mean a different tractor, or spending a few days cutting better access routes where sideways operations aren't as prevalent, or whatever.
