Unfortunately, steering boxes can not be fully rebuilt, and typically are never as good as new when they are "rebuilt", because the worm gears wear and are not available, and when they wear, the steering will still have play, but some people try to reduce this play by over tightening the adjustment. This being said, a steering box that has play in the bearings will have less play after they are replaced.
If one wants to properly rebuild a steering box that has worn gears, they can send the gears out to have then welded and refaced, however, this is a fairly expensive operation, but it shouldn't need to be done again until long after most of us are dead and buried.