[dsraven]

frame boxing is done with flat steel. there are a couple of ways of doing it, one with the steel tucked inside the C channel of the original frame and one with it sitting proud of the C channel. either way you need to start with a clean straight frame, bevel edges and leave room for the weld bead so you get full penetration, especially if you plan to grind it all nice and smooth like it was a tube frame from factory. it is a long process because you need to weld short beads and then allow the welded area to cool so the frame doesn't warp.
if you were to build a frame table then you might as well start from scratch and build a new frame from square tube. buy an assembly manual and the frame dimensions will be in there.
I have seen some guys box in just a section here or a section there and that may be fine but it could also be a recipe for a cracked frame at the ends of the boxed areas. depending on the HP and use of the truck. frames try to flex under power and even just going over bumps etc, and they will find the weak point to allow that to happen, so, a short section left unboxed, like for a stock cross member to fit through, would become a great area for the flex and ultimate crack to happen.
anyway, just saying, be carefull whatever you do or it will just be a lesson in how metal bends when it is welded. it would be a conversation piece though....