Here are some photos of the installation of Mid-80's to present Mazda 626/929 REAR retractable seatbelts. These are installed in a Silvermist 1962 GT Hawk, but I have installed them into several other Studebakers without any trouble. The retractable unit is entirely encased in a plastic box..... and measures about 4"x 4" x 2" tall. They mount with 2 bolts through  nice mounting flanges on the sides. This allows for a large "footprint" on the floorboard making them very strong. Glueing matching carpet to the outside of the box makes them virtually disappear into the floor. There are several different latch mechanisms used by Mazda. (Ford latches may also work)  I usually try to find the latches out of a bucket seat car, as they are usually longer for better fit into the taller Studebaker seat positions. They are either mounted by a plastic-encased belt or a cable, both of which can be painted/dyed to match your interior. There isn't a huge selection of colors available, but usually a complimentary color can be found.