As the saying goes "...make lemonade". I'd figure out how to make sure main beams were resting on the frame and hinged on the back. Get a small hand pumped hydraulic jack to lift.
Even 2X8's sounds excessive. I'd probably build a 2X4 "wall" for the decking to go on. Then run 2 - 4X6's (or beeter yet some c-channel) the length of the truck perpendicular to the 2x4's against the frame to facilitate the "lift". Finally run cross angled 2x4's from the 4X6's to the the outer edge of the 2x4's. That last step will not add much weight, be cheap and most importantly add a tremendous amount of strength. All the weight on the outside of the bed is directly transferred to the 4X6's. This believe it or not would probably be much stronger than simply building a 2x8 wall. You're adding angles and structure to the design. More cutting but the end result is more desirable, less weight and more strength. I can draw it up if you can't visualize what I'm saying.
As far as the decking, either three quarter plywood or decking. Be sure to use metal brackets at all joints.