As for the retractors, in 1966, this was "newer stuff". The aftermarket had some you could clip onto the outside belts, which were "open". By comparison, the factory belt retractors (as on our '66 Newport) a vinyl-type casting which protected both the belt and retractor as it laid on the floor between the rocker panel/sill plate and the seat base. Color-keyed, of course. The retracted belt went fully into it, leaving the metal buckle "male" end exposed for ease of grasp.
Probably the best source for these might be an older salvage yard? I suspect that by the time the shields became worn enough to possibly need replacing (a non-wear item), the cars would have been scrapped anyway. But for those vehicles which went to the salvage yard earlier, due to collision issues, those could be your best sources. As with any trim item, the northern sections of the hemisphere might be better than those closer to the equator?
The "webbing" in the belts was similar to what we have in the '70-era vehicles, but a bit stiffer and possibly thicker. One interesting thing regarding seat belts, back then, was a recommendation to pull them out every few years and wash them. Why? Allegedly, if any grit might get into the belt, the friction of it against the threads could degrade the threads enough to make the integrity and strength of the belt a bit degraded. Which means "strength" and their agility to keep the passenger "planted" in the seat in a crash situation. BTAIM. Never did hear of any injuries caused by failed seat belts, even back then.
By observation, the factory belts seemed much more substantial than the aftermarket "woven belts" back then. The ones in our '66 are still in there. Never been washed or whatever.
The retractors were pretty much universal, being placed more in the middle of the belt than at its end (as the newer retractors are). As with the non-retractor outside belts, they were pulled "taut", then buckled into the inside belt buckle (with the chrome "aircraft-style" chrome pull-up release buckle, and then the whole belt pulled taut with the free end of the inside belt. Everything "taut" when latched.
The belt/retractor shield bolted into the same place on the inner rocker panel as the non-retractor belt end would. As one assembly. When the bolt is removed, the belt/retractor should come out of the shield. It was "loose" On the bolt shank so it would pivot upward as needed, when it was deployed by the person in that seating position. ONLY on the outer seating positions.
I don't specifically recall, but there was probably a similar set-up for the rear seat? The shield was more hidden by the seat cushions, I suspect? Only on the outboard seating positions. Don't recall any factory retractors on the center seating positions.
I don't have any pictures to post of the set-up, but I also suspect there might be some in the "New Model Information" publications/videos online at
www.mymopar.com or in the sales brochure sections at
www.hamtramk-historical.com? "Safety" was an emerging selling point back then, so the "luxury" of seat belt retractors was an aid to that. No need to fumble for the outside belts as they were easy to get to in their shields, which pretty much where you'd reach for them. Greater convenience was supposed to lead to more seat belt usage, back then.
Hope this might help some,
CBODY67