So a weird thing on this (at least the wife's GM product)and I know it's not completely specific to your situation. It seems that some car companies knew that safety changes were coming. I believe (don't quote me) that new federal guidelines were going to require head rests (maybe 69) and shoulder belts in the 1968 model year. In 67 GM actually had put in the mounts for both the shoulder belt and the headrests. I thought I was going to need to do a bunch of retro fitting for her 67 El Camino. Nope, she has shoulder belts and headrests for safety now exactly like the setup you have pictured. They mount at the B pillar about halfway (which you don't have) up at the stock location. They work great, except they are a little catchy when you first pull them out each time. You have to be easy and deliberate. I also know that Mopar had a weird way of dealing with the shoulder belts. At least in the 68 Super Bee and the 70 Duster that I have here. It is a separate belt and mounts at the roof line and is tucked/held up with clips. I don't think mom ever used them. My convoluted point is. There should be stock belt available to you for any year 68 and after. Barring that. I'm using aftermarket belt like your picture and I like them for the wife's ride. They look like they should be there.