I'm thinking a relief cut, bend slightly, then re-weld. That or put a hinge so that it can float better. It would need some kind of lock for transport.
The plane needs a hinge, needs to float on its own, and the only thing it wants from the ATV is pulling power. You don't want the ATV crossing a ditch, to lift the blade off the surface. Just illustrative, I don't believe this would happen, but I think you want the blade to be completely ignorant of what the ATV wheels encounter.
Aside from that issue, you're pulling from the center of the ball, (up a foot or so high) while the landplane is resisting this pull by its ground contact. Which creates a 'moment' (or another word used is torque) rotating the landplane forward, and tends to dig the front in, and raise the back of the plane. As you reported. You can add weight to the rear to resist this moment but that will add difficulty to the pull.
A detail to pay attention to is the ball height to ground contact dimension. You could lower that a little by going to a clevis type arrengement, or maybe lower it further by inverting the ball in its receiver and flipping the trailer hitch upside down on the landplane.
On the landplane, consider changeing that drawbar to hinge very near to the ground. I can't type without armwaving too, so if you don't understand what I typed, unhook the ball hitch from the ATV and tack a couple chains right at ground level on the skids, and pull it around, and you'll understand what I am trying to type. A hinge will work if its near ground level, and perhaps a longer drawbar will help too. You can experiment with chain length. Longer will tow better but length needs to be balanced against maneuverability.
Maybe it doesn't need a hinge if the hitch (and ball) are invert-able (downward), then maybe the ball rotates upward (in the receiver) for transport, down to grade. Which will also add down-force to the front of the ATV in a hard pull. You don't need ground clearance while grading, only while transporting.
Anyway, this is all typing across the continent - you will figure out what to do I'm sure. As I said before I'm inspired by your project to build one and very much appreciate your work reporting the weights and the assessment of what an ATV will pull.
