I've built beds. Just look at beds to get idea of layout. What I do is...get treated lumber in 5/4" decking and cut strips to lay on frame rails full length. Cut the holes in it to lay flat over the rivet heads in frame, so the board lays flat. Buy 5" or maybe 4" channel to use as "rails" to set on top of wood and follow frame rails. Get them long, like 12 to 16ft or whatever you had in mind for length of bed. Use any tube or pipe to make a couple of x-members between them, usually at least three, to hold it all together as you build. Next, using more channel, weld a frame outside edge as big as you want bed to be, so it sets on the channels on frame at front and back. Now run smaller channel, like 3" accross the main rails an into outer channels and weld to them. Weld those smaller channels to the main channels. I like to make my outer channels "mitred" or 45 degrees for finish look. You could cut corners out and make them angled, not square and you can make a "dovetail" outer end if you wish. Buffet the rais to x-overs with little triangle plates for support. Use treated tongue and groove 2x6 boards for deck, painting them after they dry/shrink some. I used carriage bolts to bolt to x-overs every so often and ground down the edge boards and the ends of each to fit under channel lips. You will have to put the last end channel on last after boards if you make it flat. I can send a picture or two of what I did on my last truck. They are not that hard to make, provided you have welder, and good cut-off saw. Once you have the main frame set-up and tack welded, you can remove from frame and weld better or turn to weld and paint off truck. Set bed back on truck and hold whole thing down with long U-bolts you can get at spring shop or some places can make you some to your measurements. They are fairly cheap. About six is all you need, two on front, two on back, and pair in middle or maybe two in middle if you think needed, I don't.
Check out
www.trailerplans.com for bed ideas for trailers, they are about the same.
