Seems odd to consider all that work and the not consider the issue of safety. Is the new bed going to fit properly with the brake lights and bumper or would the bed extend beyond those? If it extends it may not pass a safety test if your state has those. You might also be liable if an accident happens and it was found you altered the vehicle and made it unsafe. Not sure why you would go to that expense / trouble rather then just trading it in or selling it. Just get another truck that has a flat bed and you are good to go.
Would be neat to see one done. We have a friend that we camp with and he did it to his 1999 Ford f350 Crew Cab Truck. When we camp we use his truck for the buffet when we eat. He said if we did it to my Sport Trac it could be the Salad Bar (LOL)
Steve's (swshawaii) link shows my Sport Trac w/o bed.
So now my project IS a flat bed but it's on a short wheelbase F150. LOL, won't be much longer than on for a Trac. It'll end up being 6'W x 6' or 7' long, whichever makes it look less odd.
6 or 7 feet will look way better than 5 feet especially with the long cab of an ST. I love the salad bar comment lol. As far as having a flatbed though, they are really useful on wheeling trips. One of the guys in our offroad club has a flatbed on his ext cab yota and it's great for gathering wood and doubles as a giant table in the site.
The only problem with that long of a bed is it will look disproportionate. To look right, IMO, the rear axle should be center of the bed, if not farther to the rear.
My bed is sitting on a parts truck right now to keep it off the ground ad you can see what an 8' bed on a short bed F150 looks like. Too much overhang for my liking.
Plus if you're wheeling in the bad rough terrain (which I won't) that much overhang will surely hang you up.