I do not think you are going to find a kit for this. Also, do you plan on doing just the floor or building a whole new bed?
You will need to consider that the bed sides, bulkhead, and floor are all one piece. This means that the sides and the front of the bed are attached via the floor, and the cross members under the floor. If you replace the floor with a wood one, you will need to find a way to attache the bedsides and bulkhead to the floor, and the frame, This would take a lot of custom fabrication.
You also need to make sure that you retain the original rigidity of the bed, the bedsides will no longer be completely welded to the floor and then attached to the floor. So this can be one of the bigger challenges I see.
As for mounting, I would simulate the original design for simplicity. 2x2 boards for cross members, a through hole, and bolts. You may even be able to use factory replacement bolts, but they might be too short. Then have the main planks run the length of the bed, same direction as truck driving.
If you want a wood flatbed, or with stake sides, should be fairly easy. Only tricky part on a flatbed is the mounting of the filler neck, but the can still be done.