I installed a set of seat belts plus shoulder belt about 15 yeas ago. I think they came from Seat Belt Pros. I don't think they are still in business. They are like most of the aftermarket ones, an aircraft type buckle in the center with a self retracting inertia type on the other side. They came with mounting hardware. They are bolted to the floor behind the seat and I added a square of 1/8 steel plate to the underside the floor for a better anchor. For the shoulder belt anchor I welded a piece of 2x2 square tubing to the rear body seam behind the seats and fastened the shoulder anchor point to that. This results in an anchor point that is maybe a little low but is certainly strong. Probably doesn't matter but I have bucket seats. I'm not one for taking a lot of pictures, If you need i can see if I made some.

Harold Wilson
41 Chevy 3/4 Ton