I had a 12' box on my d300 and built a rugged wood bed. I know, I know, I am a frenchman but I built this bed years ago and it has served me well enough. I took 2x4's and laid them flat across the frame rails front to rear using the original "levelers" which were basically a 'z' configuration with small diagonal gussets bolted to factory frame holes. The spread I believe that is universal is 32". The bottom 2x4's were through drilled and lagged through frame. I then put 2x3's from side to side long enough to clear the duals and the sides were 2x4's. I precut everything and stained it black. The 2x4's are flush on the bottom so they stick up about 1". This accomadated the 1" oak I salvaged from a local machine shop's steel rod shipping boxes. 1" oak and is very durable. I used my head and thought it all out and it works great, is low as it could possibly be and will last indefinately. I guess that you could use 5/4 x 6 planks with the same results but you have to buy those and th eoak can be had for a half-dozen coffees! I have to redeck it soon but it is still used every day to haul anything. I have an 7'3"?x12' bed and I took one sheet of plywood (1/2" 5 ply?) ripped it down to 16" giving me 3 8' lengths and made a 3 sided box and a lift out rear panel to haul sand, stone brick, etc. with a brace cross wise at 4' to keep sides from bowing out and a light chain at the rear. I know this sounds "French" but many people have leaned on the bed for an hour before they realized that it was wood framed. I have had 3 tons of wall stone dumped in with no damage at all (use one sheet of plywood layed flat on bed to protect from any heavy materials being dumped from a loader and to keep sand from slipping through the spaces between boards. Keep planking in a dry garage for a few days to "shrink" it and be aware that if you fit it tightly it WILL swell when it rains. I've had a $45,000 Harley drag bike on there and even once drove my built CJ-7 up there to take a drive and win a bet but THAT was scary (2x3's) and I am starting to see more and more of those "boxes" on flatbeds around my area. I can haul 1 1/2 yard of stone and bags of cement on the sides plus mixing tubs and wheel barrows on the empty bed sections. Sorry for the long winded reply but it works great as long as you dont want it to dump. Plus it's light, I put it together and lifted the bed frame myself from the saw horses to frame before I screwed the decking down. I have full DOT lighting, just had to run ground wires. Total cost was around $100 and after wear and tear beats up the side 2x4's I just hit it real quick with some flat black rattle cans as well as the frame and it looks really clean. Have fun, works for me and I work the hell out of this truck.
