You may have trouble finding them. There are post with alternate ways attaching them to the bed slots. One was to use Carriage?? bolts with a head a little larger than the tract in the bed rail. File a flat edge on both sides of the head so that it slides in snug in the bed rails and cut to desired length. The flat edge keeps the head from turning when you tighten either a hex or wing nut.
1. I would suggest a thick fender washer slightly larger in diameter than the inside of the track with a hole that can be tapped to 6mm so the D-rings will thread into it. Cut flats on two sides to keep it from turning. If you make several you will have some to use in the side tracks as well.
2. Maybe it is time for me to approach the Machine Shop tech class at the Jr College about reproducing some of the originals as a project. The D-rings are the hard thing to reproduce.
Here's what I did to make a rail that fit in the slot:
I went to Lowes and purchased Hillman 3-ft x 0.75-in Aluminum Metal Flat Bar.
I purchases some Bleacher Bolts online because they have a flat head.
I marked the holes on the bar, drilled them for the Bleacher Bolts.
I slid the bar with the bleacher into the rails and secured it with washers and wing nuts.
This is what worked for me.
Sorry, I forgot where I got the Bleacher Bolts from online.