I'm gonna go against the safety slugs here, as long as the two bottom bolts are secure and tight, no slop between the box and frame, you should be okay for road driving at slower speeds. I believe that a 2wd box only has two bolts holding the box to the frame...
BTW, if you are getting the top bracket welded in, you should also have them plate it at the same time. They are going to have to take the box off to weld the top bracket into place, so you might as well plate the front and back while it is off.