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.)