1964 Dart seat belts

Get under the car and you should fairly easily find the anchorage reinforcement plates where lap belts would get installed for the rear seats.

Most of the factory belts at that time were made by Pontonier, but nothing stopped dealerships from buying whatever random brand of seatbelt they could get cheaper than the ones available through the Chrysler parts system.

Spend some time and thought on getting your seatbelts right: lap belts are worthless and 4-point harnesses are very dangerous. Bunch of belt upgrade info and links in this post (read past the non-belt-related stuff at the top).