I think you are talking about the piece that bolts to the back corners of the car, left and right, by the tail lights, right? If so those are called quarter panel extensions. They are held on with 3 or 4 bolts that you access from inside the trunk, right? It's basically the pieces that form the back corners of the car? If so, ask for quarter panel extensions or end cap.
Are you sure you know what you're talking about? I own '65 and '68 Mustangs and have been working on Mustangs for nearly 30 years and I don't have a clue as to what you're talking about.