I see shorty SRW flatbeds often. As long as you have stake pockets for sideboards and a tailgate, then they work fine. The sideboards and tailgate don't have to be any higher than 2x4 or 2x6 lumber, but without at least some sideboard and tailgate, you'll have cargo rolling off the bed too often.
You can mount underbed storage cabinets on the passenger side. With a shorty, there won't be room on the driver's side for much of an underbed cabinet after you make provision for the fuel filler tube to be accessable. You can also mount tool/storage boxes on the top of th bed, across the front and down both sizes, if you choose low-profile boxes not more than about 18" or 19" high.
Your local truck body retailer probably won't have a shorty flatbed in stock, but he can order it from several different suppliers, including the big names of Omaha Standard, General Truck Body, Knapheid, Reading, Stahl or Supreme. Or there's probably a local truck body welder-upper near you that your truck body retailer can order from.
One caveat is that the beds for pickups and chassis cabs are much different. One won't fit the other. So the shorty spec should be enough warning to the supplier that you have a pickup with the bed removed and not a chassis cab. But just in case, spell it out for him.
Most decent-sized towns over about 100,000 population should have a local truck body retailer. Look in the yellow pages under truck bodies.
You want not more than 48" distance from the center of the rear axle to the back of whatever sticks out behind the bed - usually the rear bumper and the receiver. Then you can mount a fifth-wheel slider hitch 2" in front of the center of the rear axle and still be able to turn sharp corners without trailer to rear bumber contact or - with the hitch slid back - without trailer to cab contact. So that means about a 7' long bed is the max you can go for if you need to tow a fifth-wheel trailer.
If your trailer will be a tag, then you could go with an 8' bed as long as you
always use a weight-distributing hitch tightened up to transfer at least one third of the hitch weight to the front axle of the truck. If you don't want to use a weight-distributing hitch for gross trailer weights less than 5,000 pounds, then limit the bed length to 7'.
SRW pickups have a weight problem. Your GVWR ain't much. And steel flatbeds are heavy, which doesn't leave you much capacity for hitch weight. So if you want to haul normal trailer loads, then get an aluminum bed instead of steel. A lot more expensive, but a lot lighter.
