Did it on a Duster I had in another life. Seat belt strap bolted to the track mechanism attached to the bottom of the glass. Fold the outer end over and sew. Install a heavy duty snap and install the male half of the snap in the metal below the door panel. The door still had the OE arm rest. I ran it just behind that. Pretty simple. I wasn't trying to make it a perfect copy of the '68 cars.

If the results don't match the theory, change the theory.