I ran the wheels you're talking about with the regular GMT400 trim rings and thought they looked just right. Obviously a matter of personal taste! They're only slightly different from the rings that come with those wheels.
The original trim rings for a 15x8 truck Rally are wide and cover over the "step" in the hoop of the wheel. To me it's an old school look and for a newer truck seemed a little out of place. They look good, but it wasn't my taste, and also I was missing a couple of them.
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I went with the regular GMT400 trim rings which are a little narrower. Also ran an Astro van center cap which to make them fit correctly takes spending a few bucks; they use the regular externally threaded lug nuts from a 400 but you have to use the early black plastic lug caps from 89-90 because they're deeper and get enough thread engagement that the caps won't fall off. I guess some have made them work with the more common 91-up lug caps but they didn't feel secure to me.
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Sorry for the giant pics, figured it would be best to show the slight difference in them.
Richard