[dsraven]

are you talking about a fully boxed frame then?
I think GM relied on the box bed as partial strength possibly, although they did sell the cab and chassis only, not sure if they sold them as a 1/2 ton truck.
I think bolting the box onto the chassis helps make it more rigid in the rear half. I have also seen some boxed frames that have cut outs around the area of the front of the box. this, to me, could cause problems, especially with high horsepower units, because the rest of the frame is pretty solid and the section left unboxed (sometimes because there is a crossmember in that location and the cross members were not replaced and attached to the boxed section) becomes the soft area that will do the flexing, and therefore possibly become the area that stress cracks or stays in the flexed position.
just my 2 cents worth, and I am a long way from an engineer. haha.
with the MII cross member the front area would have had short boxing plates, correct? there is a whole school of thought around the different ways to weld in the boxing plates to make them look like they are part of the original frame without grinding away a lot of the weld. I suggest to have the frame on stands, cleaned down to bare metal in the welding areas, (as well as the boxing plates), check it straight and square BEFORE welding in any boxing plates. also consider how you will work around the existing cross members etc. you don't want to cause other problems by adding stiffness. also, do short welds in different areas so you don't heat up the frame and end up twisting it.