I've never used a ballast box, though I've sure looked at them... On one tractor I have a 96" box blade that provides over half a ton of weight. I'd probably make some saddle type weights like we used on large plows when I was a kid to fit over the upper edge of the box blade before I'd buy a ballast box for this tractor.
When I added a built-on backhoe (not a three point mount) to my 4wd John Deere 4210, I was adding well over 1,000# of ballast on the back of it. It didn't raise the center of gravity in this case as the JD 47 backhoe is very heavy at the base. The amount of weight this placed down low on the rear of the tractor has done two things to the tractor besides making it much heavier, 1) lowered the ground clearance by several inches 2) gotten us stuck more often as it is heavier and the weight distribution has shifted back considerably lightening both the wheel pressure and traction of the front end.