There's also the DC style front spoiler for the Dusters, although I can't find anyone selling a replica at the moment
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And the rear DC spoiler, which is being sold currently. I have one from this seller, it's a pretty accurate recreation...
70-76 Plymouth Duster and Dart Sport Showcars Rear Spoiler | eBay
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And for a more air dam version there's this...
Awesome A-body air dam for under $30!
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It's off a Ford Ranger. This is obviously a Dart but since it's mounted underneath it should work about the same on a Duster. Probably do a better job than the Camaro spoiler since it has better coverage. Not sure it looks as good but if what you're after is aerodynamics it's probably the better choice.