These are the ones I used;
3 Point Retractable Seat Belt with Push Button Buckle
I used the 12" sleeved buckle with the 8" drop in the 140" length. I welded mounting tabs inside the rear side panels and bolted the retractor to the tab with the retractor mechanism facing towards the inside of the car, there isn't room to put it in facing towards the inside of the quarterpanel.. I then ran the belt and drop hanger up through the hole in the side panel that originally had the ashtray in it. I used the seatbelt guides that originally went on the carpet in the 70 Cuda applications to finish out the opening.
You have to cut the guide to get it around the belt. I used a Stanley knife with a new blade to cut it and once mounted you can barely see the cut. If you look close in the picture you can just barely see it. If you cut it with any kind of a saw you will have a visible gap when it's mounted. The guides have a rounded boss for the screws on the underneath side that you have to file back a bit to fit the ashtray hole. I used some sheetmetal speednuts from the hardware store with black screws to secure the guides to the opening in the side panel.
The other end of the belt bolts to floor where the factory retractor originally bolted. You definitely need the 140 " belts and the 8" drop. I would have went with a 12" drop if it was available, the shoulder part sits kind of close to your neck with the 8" drop, pretty much the same as the factory shoulder belt did. You will probably want to flip the belt where it goes through the drop bracket so the belt lays right in use. I did this by folding the belt diagonally and pulling it through the slot. It's tight but it can be done.
This works great if you want a modern three point system. It kind of hangs in the way of passengers getting in and out of the back seat but I rarely have anyone riding back there anyway.
