I'm rebuilding the sliders for the bucket seats in my 1970 Dart. Inside each track there are two sets of bearings (one on either end) and the set consists of a roller that takes the weight of the seat and two ball bearings that allow the two parts of the slide to actually slide. Here is a picture taken during disassembly:
The roller and two ball bearings are held in place by a metal cage or cradle:
There are 4 cradles per seat or eight altogether and half are mirror images of the other half. One of each kind was mangled in the track when I took them apart (no wonder the seat didn't really slide). I am looking for one of each of these cages (all the ball and roller bearing are OK, but nobody sells these parts, that I can find). A poster here says they are the same in the same style of bench seat sliders, but I don't have any access to any trashed seat sliders here. If you have some of these, or know of a source for new ones please send a PM. Here is a picture of the type of seat slider: