1970 GMC C2500 rear axle hub diameter?

[tow rig]

I have a 1970 GMC C2500 (2WD) that I have bought aftermarket wheels and tires for. The wheels are 8 X 6.5 bolt pattern. The center hole in the wheel is just under 131mm. If I bolt a wheel onto the hub of the Dana 60 full-floating rear axle, the wheel has a significant gap between the center hole of the wheel and the hub of the axle. The axle hub is difficult for me to measure despite having a digital caliper and 4-5" micrometer. The studs get in the way of my making an accurate measurement. Does anyone know the diameter of the hub at the wheel mounting surface? (I will need to order some rings to fill the gap between the hub and the inside of the rims center hole if I want my rims to be hub-centric.)

[jerry moss]

your tapered lug nuts center the wheel to the mounting flange of the hub. you must be thinking of a Ford truck that uses hub centric wheels.

[tow rig]

Nearly all factory installed wheels are hub-centric. The 1970 GMC had steel wheels that mated tightly to the hub. I know that many aftermarket wheels are designed to be more universal, and center off of the lugs, in order to have a single wheel casting be useable with several applications. My opinion is that lug centering wheels may be less than ideal, especially for a HD truck that will be carrying heavy loads, driven in rough terrain, abused with dragstrip launches, and/or towing a trailer. Anyway, it looks like my question is turning into something that I haven't asked. Are hub-centric rings needed for aftermarket wheels?(This is a simple google search that I have already done. The jury seems to be about evenly divided on the subject.) Thank you for sharing your experience.