Do you have clearance to lower the front any? Proper front end alignment can also affect/help top end stability.

Something integrated into the bumper, straight down with a small lip facing out would help, but would involve a lot of fabricating and take away from the classic Duster look.

I built a simple, angled and reinforced aluminum plate front spoiler on my old street challenger. But that was before I ever went to the drag strip or went that fast.
It can get complicated and if it forces too much air under the hood, you could have problems keeping it on.