Tailgate & back bumper.
Pic 1, This is the right side of the tailgate up close and sort of personal. What I want to attract your attention to is the right side hinge area. Notice there are 3 bolts there, two offset in the hinge, and one below the hinge. Also notice below the black painted rear crossmember that all 3 of the bolts pass through you can see a small strip of reddish metal. That little chunk of metal is very important. The bed sides are sitting on pieces of 1/8" thick 1" x 1" angle iron that is bolted to the sheet metal bed floor. That angle iron is all the support the rear bed sides have. At the front of the bed, in addition to the angle iron, the bed sides are also bolted to the full height front panel on both sides, and that front panel is welded to the floor. It should be pretty obvious that if I want a tailgate that I can lower, I'm going to need additional support for the bed sides. On nearly all of the pickup beds out there, the back corner of the bed sides are welded to the rear bed crossmember, and many have an extra brace below the rear crossmember that extends to nearly the bottom of the bed sides. That attachment is what keeps the bed sides from flopping around, for the most part. There is also nearly always some sort of a catch bracket on the upper part of the tail gate that captures the bed side when the gate is closed as well. as a case in point, when the tailgate is open, the only thing besides how the bed sides are attached to the floor, the only other thing that keeps the bed side solid it that attachment at the rear crossmember and the brace below it. If you open your tailgate and can shift the top of the bed side in and out, you need to look at the condition of that rear crossmember and the brace under it and get it fixed! That problem happens faster on pickups that have no tailgate, or a truck where the tailgate is often open because the extra catch for the bedside on the tailgate isn't present.
So back to my truck, my bed sides are bolted to the rear crossmember. The little sliver of the red is part of a brace that is welded to the bedside from the bed floor to below the rear crossmember and it is braced under the floor to the bed side. That brace is tucked into the channel of the rear crossmember and the bolts are actually holding the crossmember and the brace together, the hinge location was planned to take advantage of the top two bolts. The other bed side has the same brace.
Pic 2, Lets build a back bumper! Of course it started with a cardboard pattern, then I transferred the cardboard pattern to a sheet of 11 gauge metal, then cut the 11 gauge with my plasma cutter. 3 pieces were cut, a strip 4 1/5" wide by 64' long that would be the curved bumper face, a 64" long piece with a single curve, the thinnest part, the ends are 4" wide and the center is 8" wide that is the top surface, and a 2nd curved piece with a matching curve, this one 2" wide at the ends and 6" wide in the center that would be the bottom piece. Who here would believe me if I tell you that adding a consistent curve to 11 gauge (1'8" thick) that is 4.5" wide for 64" long is a bit of a challenge without a roller? You kind of get it close by forcing the piece between, under or over some heavy stuff. Then you clamp the top piece of metal to a heavy bench (or in my case, some heavy steel saw horses), then you mark a center line on both pieces, and start at the center and clamp the curved front piece against the cut curve of the top piece with anything you can find that will hold it in place and square. You put a tack weld on each side of your clamped "movable fixture" and move towards each edge a few tack welds at a time as you move the "fixtures" and the top plate to maintain support. Sometimes you need to add a couple more fixtures so you can force the facing plate tight against the curve. at the end of that day, I was wore out. Once the top and the face was tacked together, you repeated the same deal with the bottom piece to the face plate. It was easier because the face plate was already tacked to the top plate at the curve. The hardest part on the bottom piece was keeping the the edges square and the distance between the top and the bottom plate the same.
Pic 3, This is pretty much what I was looking for. From this point, the top and bottom edges was welded solid on both the inside and the outside, and the edges were slightly rounded. Any defects in the weld were fixed. then the ends were capped, and bracing was added in the center so brackets could be made to bolt the bumper to the frame. I bought a new class 3 hitch (3,000 -5,000lbs capacity, it is a Dakota 4x4 chassis, not a full sized truck frame) for the truck, incase I ever needed to tow anything. The hitch was bolted to the frame, then I made bumper brackets from 1/8" 1.5 x 1.5 angle (2 per side).
Pic 4, The hitch and the bumper bolted to the truck, notice the tailgate is still clamped closed. There is 6" between the bottom of the tailgate and the top of the bumper. The tail lights and the license plate will be put on a panel in that space.

This is about where I tell you I just have never found a tail light set up on step side beds that I like. The fenders are too far forward to hang the lights on them on this truck, and the factory style brackets and lights that hang off the lower stake pockets always looked to me like some designer designed the back end of the truck then thought "OH Crap! I forgot to put tail lights on the truck, just hang these there." The problem is, I'm not doing any better myself...