Modern seats and shoulder belts
I put front shoulder belts from Dodge vans into my 72 Demon and 73 Dart Custom. The van belts are long enough for a two-door and there is enough room on the inertia reel for it to work on the 4-door, too. The inboard link is almost too short for use with a bench seat, however. The reel has to be mounted parallel to the ground. Otherwise, it will not lock up when it should. I used the stock two-belt system's mounting points. I did make some modifications to the belt mount plates in order to make things fit.
When looking for belts in the local pick 'n pull, watch for cuts and frayed edges on the belts and vehicles that have been in frontal collisions. Before mounting the belts, I cleaned them. It was amazing how dirty 30-year old black belts can get. The cleaning also gets rid of stuff that had been ground into the belt and could compromise its effectiveness.