75 dart sport front bumper interchange

No, Dart has the wrong contours for the grille because Darts have a more obvious "beak" and do not interchange bracket-wise anyway (early vs late). For both Darts & Dusters, the most obvious external difference is that the later bumpers have extra mounting points near the top on either side of the license plate area for a total of 10. I've heard that 1974 was a transition year but the ones I've seen had the standard bumpers & brackets with a total of 8 mounting points. The later bumpers have a VERY heavy, contoured inner reinforcement that fits between the actual bumper and the "shock absorber" brackets on the frame.

Don't forget to check out '73 - '74 Plymouth Valiant & Scamp bumpers as well as Duster.