If you are not wanting to always have your box blade attached, I would go with the heaviest liquid ballast you can, such as Rimguard, and probably add some weights also. I think you can add about 330# for rear weights and the tires likely would be 600# or so more, so 900+ pounds which should be enough to get by if you do not have the BB attached. If you did you were certainly be fine. John Deere might argue for more, but I think this would do. I did well with the 3720 I owned in all but the heaviest material on steep inclines with this setup, so I know it works.
John M