While boxing will increase the strength and stiffness, remember that the frame will need to flex to prevent stress cracking, especially under a heavy load or off road use. That's one of the reasons that you don't see full boxing at the rear section of 3/4 and 1 ton pickups, especially where the hitch attaches, and on over the road semi trucks. They need to flex to prevent cracking of the frame. Boxing the areas such as the steering box mount and perhaps the suspension and engine mounting areas would be wise, however.
