New seat belts for 1973 Duster?

Just have your existing belts rewebbed.
Many places, such as Seat Belt Planet, Seat Belts Plus, Andover and others offer this service.
That way, you know all the bracketry will fit, the hardware still appears as it should, and you can specify any changes to the length. Just be careful how much you add, as the retractors will only accept a given amount of webbing without binding- you may have to consider lap extensions, which are also offered by many of the same suppliers.
Installing them in the retractors is done when they are rewebbed, as the folds at the end have to be sewn in after the webbing is installed in the retractor.