['63GENIII]

Bunch of hooey. If I understand correctly, there isn't much difference between the frame rails of 2wd and 4wd. Once you remove the front crossmember, I would really look into a crossmember to tie the frame back together under the engine mounts but that might be about it. My '63 has never had a crossmember in that area and although Ive always wanted to install a support there, Ive never really seen the need for it even with the new suspension, drivetrain and big block. Boxing the frame IMO, while cool looking and very labor intensive, is completely unnecessary in most cases.
Stock frame in mine has seen thousands of towing miles, many loads in the box that were well over the trucks capacity without any indication of fatigue.
Im not sure why an honest restoration / body shop would recommend welding plate to the inside of the frame. Ive seen several instances where frames are not supposed to be welded on due to the alloy that they are made of. Not sure if our old truck frames would be affected. Just saying it would be good to research completely on your own before asking a barber if you need a hair cut.