How would you FASTEN it to the truck?
Just asking questions since you have done this numerous of times? JUST MY 2cents........................Again
Usually, I weld thick flat bar-stock, maybe 3/8 or 1/2 by three or so, to the outsides of the lengthwise runners that sit atop the trucks frame, maybe three or four to the side, spaced as evenly as obstructions will allow, such that they reach to the bottom of the truck's frame; then, I put two or three 3/4" grade-8s through these and the trucks frame, using any already provided holes whenever possible.
Note that these brackets are bolting flat to the SIDEs of the frame, NOT the top flange as a factory sheet-metal bed would.
Fastened thus, a crane can pick the truck up by the front-bumper and hang the trailer, with nothing coming apart.
It is a good idea to line the runners up with the truck's frame and get them where you want them, then make/weld-on/bolt-on these brackets with a couple of flat-washers between each bracket and the truck's frame, then securely tack-weld on at least four of the joists, then remove this from the truck and finish building the flat, turning the whole thing upside-down, side-ways, whatever makes what you are doing easiest to do right, then sand-blast, rust-proof, prime, and paint before mounting on the truck.
The reason for the flat-washers/spacers between the brackets and the truck-frame is to allow a smidgeon of clearance for whenever the flat is installed or removed; you can try to slide the washers between there when installing, or leave them out and let the tightening of the bolts/nuts simply pinch things together; no one will ever know the difference.
With a little ingenuity, and whatever odds and ends you might have laying around, a rotissery can be built such that it can be fastened to both ends of the flat, allowing you to turn it upside-down and back easily at will.
I always weld a BUNCH of single chain-links, at least one-per-ten-inches, in the hidden inside corners of the channel-iron, such that wire can be routed through the links in any direction necessary.
I have seen (and used) short inch-long cut-offs of tubing, both round and square, but the smooth rounded chain-links do not have the razor-sharp wire-cutting edges that the cut-off tubing has.