Sounds like it is beyond the point of no return. By the time that you welded in enough structure to support things, you might as well do a flatbed or something similar. If you want to keep the stock look, I'd just start looking for a bed. If you need something back there until you can find a bed, I saw a guy that just bolted some 2x6s across the bed mounts, nailed a couple sheets of plywood to them, and fastened the taillights to the underside of the back. It isn't going to be permanent and you're not going to be able to carry anything back there, but it will keep the cops off ya' if you need to drive it around.