I have read that the front clip of a 1970 Duster was the same as a 1969 Valiant with changes to the grille

If this is true would fenders interchange?

yes they do. I have a -69 valiant front clip on my Duster -70.
Much better looking grille to me.

And i think the only difference on 67&68 valiant is the sidemarker

^9 has the small reflectors 70 has slightly larger lights.

Correct. If you care to shave the side markers, all the 1967-76 fenders look the same on the outer skin. There are subtle differences on the portion of the fender that bolts to the core support as well as tabs to support the grille.

The head light buckets are the same on the 67 and 68 Valiant fenders. They are different on the 69's.