The Duster fender are more slab sided at the bottom. The 67-69 Barracuda fender curl under and can catch the tire as it turn through it's radius.
The Duster fenders have a fender support brace at the bottom lip, correct? I know 67-69 Darts are that way. Most A-bodies without a lower front valence have a lower fender support brace.
With the lower fender support brace, you can adjust the fender outward to give you a little more room. Or you can even drill another hole forward of the factory hole and that will push out the fender even more. I did that with my 68 Dart and fit 245/60/15 with 15x6.5 rallye rims. I pushed the fender out a good 1" or so.