Just looking in on the discussion, but if you go with only steel bed you will end up with a very warped bed on your truck. It will be hard to clean and hard to keep loads level.
I would recommend a plank bed on the cross sleepers as per the original design. The outside of the planks were "wrapped" with edge planks top and bottom and bolted together. It was incredibly strong and lasts a long time. Finish with CPE finish...(clear penetrating epoxy) and go from there.
