I did say "Load Bearing" meaning that ALL loads are fed into the cage, not just those in the passenger area.
Given that the stock frame was designed to flex and twist so that it doesn't crack and break at the cross-member attachment points as well as other load application points, boxing the frame needs to be done with a lot to thought and care. Not just to boxing it, but also to modifying how all of the cross members attach to the now boxed frame rails. Merely welding the cross-members to the rails & boxing plates may or may not be good enough. I've never cared to study it so I've no idea.
If this was such a good idea, why don't the premier K5 vendors who think thru their products & engineer them carefully, like ORD, offer a kit to do this? GM's design is a masterful piece. Some of them have been in normal, even HD normal service for nearly 35 years. To improve it is going to take re-engineering the whole thing. I wouldn't be surprised if someone like ORD was working on a whole new frame based on box tube. That would be a much better way to go. It is how
Bud-Built was first started, to build brand new yota frames.