I did some research a few months ago and found out that rear 3 pt seat belts could be ordered from the factory on a first gen. I found a few pictures but cannot find it again. It's possible that it was on a camaro. I would like to add this into my car so my kids will be safer. Does anyone have any pics of how it mounted onto the package tray?
I added rear shoulder/lap belts to my coupe. Turns out the factory option included a double layer package tray to add strength. I used Morris Classic Concepts kit for front and back. The rears use a custom bracket that reinforces the critical mounting area on the package shelf. Billy is great to work with.
Check out the May Hemmings Muscle Machines and there's a 69 goat vert with rear shoulder belts. I think he added them but it was supposed to be an option.