I don't know of any Mopars of that era had anything more than a rear lap belt. My Challengers only had two in the back which surprised me. My wife had a tercel that was half the size and had 3 in it's back seat.

Check to be sure they didn't fall under the seat bottom. I have never had an old car I didn't have to pull at least 1 if not all of the belts out from under the rear seat.

The good news is that if you get the pull adjustment straps virtually any will work. If you get the auto-retractors, that were common to the outside seats, you will need the correct (or very similar ones) to your year and body style to fit under the rear seat sides.

Don't worry about the lap belt only. They will be fine but if you really want to anchor a shoulder strap or car seat top anchor there is plenty of metal in the rear deck for one to hold in a child. It wouldn't be hard to adapt one from a new car just be sure the belts won't cut or fray on a sharp metal edge.