If the boat is designed and built to the class weight, i.e it is supposed to have a 350kg floaty bit and 550kg of ballast, and it always sailed with the ballast as part of its usual sailing trim, it's probably within the bounds of the rule. This is provided the ballast doesnt swing outside the box, and the beam and length of the overll structure dont exceed the box limits, particularly diagonally. Your box cannot be a parallelogram.