First, I assume your hubs are 2.75" back then they weren't designed in metric, just an assumption.
Most of the cool stuff on the car is metric now.
The 12" Camaro slotted rotors up front have 12mm studs and a hub that would accordingly be measured in millimeters.
The fast-ratio Saginaw steering box is metric-spec too. That isn't relevant here other than me noting that fast-ratio steering is, in my opinion, the #1 modification/upgrade to make a Chevelle steer like something that just drove out of a Chevrolet showroom. (I have the Jeep steering system in mine, modified to connect to the original Chevelle pump)
The axles in the original Eaton-Posi 12-Bolt have 7/16" studs, and I suppose deserve SAE measurement for the hub diameter as you suggested. Part of this hubcentric adaptation project is to give me matching 12mm studs for all four wheels. I am planning on adding a pair of hubcentric spacers in the back, which will be complemented by the car's existing lowering springs and near-future plans to roll the fender lips on the rear quarter panels. The manufacturer specifies the hubcentric diameter to be 70.5mm and state this to be correct for the Camaro upon which my car's brake system is based.
****AN IMPORTANT QUESTION FOR ANYONE HERE IS: Can anyone confirm the exact diameter of the rear axle hubs on a 1971 Chevelle SS with a 3.31 Eaton-Posi 12-Bolt? Is it 2.75" as suggested? I don't have a caliper/mic handy, yet I need to know exactly. ****
Cragar did give their wheel specs in SAE measurements, at least for the old-school S/S models, and I converted it to 90.7mm hub bore. This is close enough to likely work well, as was previously said.
If any of the rings/adapters/spacers need to be bored out, I can put them on a brake lathe and take some material off.
As far as the use of the metric system as it relates to my Chevelle; where I lack in nostalgia, I make up for in innovation and performance.
I think metrics are easier anyway. It makes more sense to understand and at the time that we are at the inch to the second decimal, a portion of a millimeter makes for a narrower guess range when the legible limits of a ruler or tape have been exceeded.
I attached pics of each type of adapter to be used. The rear would have rings on top of the spacer plate hubs. The front would have rings on rotors.
Thanks again everyone and thank you sir for your reply.