So if you put aftermarket 8 hole wheels on the center hole does not fit the axle. The taper of the lug nuts is what centers the wheel. To my knowledge all aftermarket wheels are made like this
This is true only if the wheels are lugcentric, which most of the lighter trucks are. 1500, 2500, and 3500 SRW GMT400's come from factory with lugcentric wheels, where the lugs have a 60° conical seat and the lug hole on the wheel has a corresponding taper. The center bore may or may not be snug on the hub. The lug holes in the wheel are only slightly larger than the studs, and as you tighten the lugs, they center up the wheel.
Hubcentric wheels on the other hand must have a snug fit to the hub as that and that alone is what centers the wheel. The lug holes have a flat face! No conical seat. The lugs will usually have a captive/integral washer that still spins as the lug is tightened down. A great exception that confuses some folks would be for example a GMT400 3500 dually - these wheels are hubcentric! Yet they use the typical 60° conical seat lug nuts with the external threads so you can attach the center caps using the black plastic lug caps. Note the 60° lugs must be used with the "clamping ring" - the big ring with the lug holes that has.. you guessed it.. flat face on the back, and tapered holes for the lug nuts.
To sum up, a GMT400 3500 dually uses hubcentric wheels. If you intend to use some wheels with a larger hub bore, they -must- have tapered lug nut seats so you can use the conical lugs, so they center properly. However, I'd still be nervous about their load carrying ability as now you're putting all the load on the studs, instead of the hub.
Example.. though these are off my '94 3500HD which use a 90° (kind of unique to the HD) conical seat lug, onto a clamping ring, onto hubcentric wheels - the clamping rings should be familiar to you since you have them on your dually, too.
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With that setup, the conical seat lugs can be used, but they're not centering the wheel - the hub bore fit to the hub is doing that job.
When I installed Alcoa wheels onto my other 3500HD, the '95, I had to get the lugs with the integral/captive washers so I wouldn't have those funky clamp rings messing up the appearance of my Alcoas. Also, those lugs retain the center caps at the edge of a slight bevel in the washer.
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Richard