Removing rear interior panel from 66 Barracuda
I don't remember seeing any threads on 3 point seat belts for the rear of a 66 Barracuda but there is bound to be someone that has some ideas.
You may try starting a thread asking and see what ideas come up.
By the way, I was stationed at RAF Greenham Common in the late 80's. Loved my 3 years in England.