My dart used to get a light feeling above 120, and it helped alot to lower the front, I also added a spoiler from a 69 camaro. The lower valance on the dart made this easy. The duster isn't the same, so the spoiler might not work.

The spoiler I used was ABS plastic, pretty flimsy itself, but I fastened the top to the valance, then ran tube supports to the bottom edge. I could putt the car forward from the spoiler, there was no deflection at high speed.