That winch set-up looks great. Speaking from my own experience, I'd make sure your frame is adequately strong before making the tractor too immovable.
I adapted a dozer winch to be PTO powered (for size reference, it came with 100+' of 5/8" cable). After pulling out many thornapple trees with it (sometimes 2-4 at a time), I was pulling one group of trees out while braced against an immovable object. It bent my welded-up frame (essentially the equivalent of 3"x3"x 1/4" angle) like nothing was there /forums/images/graemlins/blush.gif. I subsequently rebuilt the frame with 3"x3" 3/8" angle and extra bracing and have not had a problem, but I also try to make sure that the tractor can slide if it needs to. It doesn't take but a second or so to twist the frame out of shape. Of course, having a 20,000lb winch on a CUT is probably not the greatest idea /forums/images/graemlins/cool.gif.
-Chris