I'm the OP on this and after all the great ideas, here's what I'm doing. I've cancelled the idea of a long shelf because it won't work and I don't think was ever designed that way.
*I'm going to make a small shelf by sliding in a piece of flat aluminum plate with locating pins on it and setting a wooden box on it that can be easily removed. It should not move around.
*Chains will go into cut-out HD laundry jugs because it doesn't take much room and easily removed. Great idea. Plus, my chains will smell clean and fresh.
*I'm getting some postal plastic mail tubs and I have some plastic milk crates that will hold other stuff. Also, Harbor Freight sells a great plastic "milk" crate thing that really works. Ratchet chain binders will fit in crates.
*Hydraulic jack, big tools go in plastic milk crates or loose.
Everything is easy to get at because either containerized or easy to remove. I carry a plastic step-stool to get in the bed so no reaching for me.
If I wanted a long shelf I would cut a piece of 3/16" plate aluminum almost the length and almost as wide as the truck box width on the inside. You could wiggle it in. I would then add a series of locating pins** and set wooden boxes on those pins. It would limit underneath storage but be easy to remove.
I'm not a contractor but a rural guy with farm things and this fits me perfect. If I was a contractor I'd get one of those 4' x 8' sliding tool systems for the back and sides of the truck.
**The locating pins I would use are the ones with the detent balls in them like this:
