welcome to the shop Panhead, you'll find the shop manual useful for mechanical things, but the assembly manual is the one you want for just how the whole thing fits together ...
these folks have lots of info in pdf's about the beds ... you'll need to scrounge some sills and skid strips, and the side angles, as well as wood of course
the basic deal is the cross sills lay on the frame, the boards lay on the sills, the box sits on it all and the bolts go thru the wood, the sills, and the frame [+ the skid strips are bolted to the sills] ... the bedsides do have an angle strip the full length welded to them, and the rear 'post' is welded to the back cross sill, which is welded to both sides, not free like the rest, and the front panel is bolted to the sides