Having had a 65 Barracuda and a 71 Duster, I'm not sure how all the numbers compared, but I would think a form fitting Barracuda cover is not going to fit a Duster very well.

Its going to get pretty tight around the roof area. The roof line on the Duster is wider and more squared, while the Barracuda was more narrow and more rounded, especially at the rear window area.

If the cover was not so form fitting, then the Barracuda cover might fit the Duster OK, but I still think it will be pretty tight. Gene