Has anyone used a wheel spacer to fill in the rear wells on a Duster?

There's your problem. THe new car wheels are all backspace and no front dish like the old cars and wheels are. Does your car have disc brakes by chance? If it does that will aggravate the issue because the front track is wider than the rear (of course) and wider than drum brake cars. Your car looks good with the Rallys I would order up a pair of 7 or 8" wides (15" if you wish) with a 4-4 1/3" backspace and rock and roll. That's what I did on both cars.

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OUR problem is the duster body is wider in the rear than your darts. We can't just make our cars look excellent with different wheels like yours do :)

Plus, if I recall, the 72+ duster 7 1/4" axle is a couple inches narrower than earlier axles.