Where are you from? I have some barrels of concrete filled already That I would let you borrow if you were close.
Your 2555 is about the same size machine as my MX5100....with a loader rated a little higher. But that is irrelevant, because your 6' bucket load of stone isnt gonna weigh any more than my 6' bucket loaded with stone. And well below the capacity of either machine. However
ballast still is a good idea because when scooping out of a pile of stone....its real easy to lift the rear of the tractor trying to "break-out" of the pile. Especially if your tires arent loaded, which I didnt see whether or not they are.
How big is your box blade? How heavy? Most of your run-of-the-mill box blades of standard duty design are gonna be somewhere around 500-700 pounds. And while that isnt enough to effectively ballast your tractor IF you were using the loader to the max....it may just more than plenty for bucket scoops 1200lbs or so at a time of gravel.
And since the box blade sticks back further than a barrel would....its less weight has more effect (leverage).
A 1000 pound barrel filled with concrete would probably be no more effective than the 600-700lb box blade. And the box blade can also be used to help spread the stone if that is the plan. (You just said move gravel so I assume you are spreading it but may be wrong).
A rotary cutter (AKA bushhog) if you have one may be your most effective ballast. As far as common implements go....they are generally near the top weight wise, if sized appropriately for your tractor. That in conjunction with how far back they are makes the most effective ballast. But due to their size and length, they arent always the most convenient.
Again, you are just moving gravel no where near the max of what your loader is rated for. But if you ever find yourself lifting logs or pallets and wanting to utilize your loader to the max....something in the form of a box or barrel that sits in close to the tractor would need to be in the 1600# range and even then I feel that may be a bit on the light side.