I may be way off base here so please feel free to correct me! I can't see how you can box the frame without sections that are not boxed or there is a need to rebuild the cross supports. The support behind the cab is riveted on the outside and top. I can not see how you could box this area without leaving a hole for the crossmember or having to rebuid the crossmember. Spring mounts and so on rely on being able to bolt to the channel frame. If this area is boxed, you will need holes in it to reach the bracket bolts. If you have made the rest of the frame ridgid by boxing, all of these openings and holes localize the stress induced in the frame. I guess my question is, if you want to box, how do you deal with cross members and other attachments that would need access?
