Whether you're repairing a truck with a mangled or missing bed or are simply looking for a little more versatility, a flatbed or modular bed might just be the ticket. Pickup trucks seem tailor made for such custom appointments. Their multi-part body structures mean that you can replace any one part with anything you want to without disturbing the rest.
The first thing you'll have to decide is exactly what kind of bed you need and what you'll be using it for. If you're looking for a do-it-all truck bed, then it's hard to go wrong with a traditional wood-floor flatbed with removable sides (a.k.a. a "modular" bed.) If you're planning to haul a car, fertilizer or other harsh chemicals, you might want to forgo the wood for some corrosive-resistant metal. Mechanics and professional workmen might be best off with a frame-and-sheet metal bed with locking side cabinets and a flat-floor bed (take a look at your local electric company service truck for inspiration). Each has its own virtues and limitations.
Before building any kind of flatbed, you'll have to give it a flat place to sit. Don't even think about setting any kind of flatbed directly on the factory frame rails. Factory rails are rarely straight, and you're sure to encounter wheel clearance issues. If you want to keep your bed truly flat, measure about 5 inches from the top of your rear wheels and weld some thick-wall 2 by 4 inch rectangular stock to the top of the frame rails to get the clearance you need. Next, weld in a set of well-reinforced cross members at the front and rear of your frame and two in the middle. Box the ends on with another length of rectangular stock. At this point, you can either install flooring made of traditional 2 by 6 pressure treated planking, diamond plate aluminum or plate 1/8 to 1/4 inch steel for really heavy applications.
If you need locking cabinets with a fairly stout center section, the solution is simpler in some ways and more complicated in others when compared to a flatbed. For a floor, you can simply weld or bolt to frame rails a piece of 1/8 to 1/4 inch plate steel about 3 inches narrower than the inside of your tires and cut to the end of your factory frame rails. Build outward from the plate floor starting with a set of 2 by 4 inch rectangular stock supports running along the bottom edge of its sides. Connect those struts to the frame rails at 16 to 24 inch intervals and on either end.
At this point, you'll have a solid foundation for framing your toolboxes, which will also serve as the bed's sides. 1 by 1 inch square steel stock makes for good framing material, and use easy to cut, measure and mount your outer panels to. You can use 1/16 inch thick plate steel for the toolbox skins, or aluminum plate if you would prefer to nut-and-bolt or pop-rivet the assembly together. If you're using aluminum, you can weather seal the panels by simply running a bead of silicone along the frame mounts before mounting the skins.
