I have a set of factory "cargo tie downs" for the bed of my '84 El Camino. I am told that it takes "jack-nuts" to attach them to the bed.
What are they? Where do you get them? What size should I get if I find such a thing?

I am not sure, but it sounds like what I have called rivnuts. Inserts like a rivet, but is hollow and threaded in the center. The size would be whatever the screw size is that you are using to attach the part. You also need a tool made to insert them. If you have a supply store nearby that sells fasteners, they will most likely have what you need.

Thank you very much for the information.
I would love to get those mounted.

http://www.emhart.com/products/pop/jacknut.asp

^ Those are a bit different than what I have used. They look like they would do the trick and don't require a special tool. The ones I have used are what Black & Decker is calling a Pop Nut.

Could these be used to secure the cargo rails which are notorious for coming loose?