Mark,
I have been skidding with a 55 HP tractor and a Norse 490 (10,200 lbs. pull). I moved up to a 65 hp. Even though the 55 had 4 WD and a bucket, the loads I'd pull out would bring the front wheels of the ground. The 55 had enough power, although I've gone up a few slops with a full hitch where the rpm's would drop a good 1400, but it didn't stall. My hitches will be 5 - 6 full lengh (branches are topped to 1.5 inch diameter) maple, cherry or yellow birch with an average diameter of 14 inches. The load pulls harder with the branches on.
Just got the 65Hp and haven't taken it to the woods yet. It is quite a bit heavier than the 55 hp. We'll see what it does next month. I'm going a extensive cut, mostly hemlock and white pine (some with 30+ inch diameters).
Bob