I'll concur that a ballast box is of limited practical use. I made mine a couple years ago out of a leaky 60 gallon air compressor tank. I've used it a few times on my former
L3200 & now
L4060. But really it just sits. I use an impliment as it does something useful in addition to being heavy. Generally a box blade or mower.
The Economy L machines can lift more than they should for their weight. The back end of the
L3200 got really light moving some hay bales that were as much as the loader could lift. I had a LandPride RC1860 rotary cutter on the back, which supposedly weighs 600lbs. That weight is far back to, making it even more effective ballast. I tossed maybe 200lbs of junk on the back of that mower for even more ballast.
The good rule of thumb is put as much weight on the 3pt as you plan to lift on the loader. By that mark the 1,200lbs bales were more than the ballast, even though it was a ways back. It definitely felt under ballasted.
Loaded rear tires help keep the rear tires on the ground, but make life worse for your front axle. Do you want all the weight over the huge dumb simple axle? Or do you want it over the small, complex, expensive front axle? Ballast on the 3pt not only keeps the back end down, it unloads weight off the front axle. Ballast in the rear tired will let you overload the front axle easier.
Think of a tractor like a teter toter with 2 pivot points. Put the fat kid on the end of the teter totter with the simple & strong axle, not the expensive weak one he'll crush.