Jay,
Nice original bed. That will clean up nice and look great with new wood.
What is missing are the long beams. A pair of 2x6 wooden beams standing on edge run from the front to back. They are attached to the cross members by stamped metal brackets. These brackets bolt to the 2 pairs of holes on each cross member and then are bolted onto the sides of the 2x6's.
U-bolts then drop over the wooden beams, down to the bottom of the truck frame and a steel plate and nuts hold everything in place.
Usually 2 U-bolts on each side for a short bed and 3 for a 12'bed.
I have a PDF of a document called "Unit Designed Chevrolet Truck Bodies". I don't remember where I found it, but about page 38 is perfect view of the bottom of your bed with the long beams and brackets in place.
I can email you a copy if you can't find it on line.
