Hy JH51, I would use at least one piece of 4 X 4 wood located between the chassis and dump box in the up position. Position the support on a chassis crossmember or frame rail and against a substantial part of the underside of the dump box. You should come up with a method of holding the brace in place, something other than the weight of the dump box resting on the support, that way it can't be accidentally knocked out of place. If you have a substantial piece of steel you could tack weld it in place also. More than one piece of wood or steel would be better, try to position the brace(s) so they are not in your way while working. Good luck, be safe.
