From my little bit of experience not all hubcaps will fit every GM 6 bolt wheel. I had a set of 15" wheels with 60's or early 70's hub caps and the factory repro hub caps would not fit for me... I also had a set of original 16" wheels and they fit the repro hub caps perfectly. check my pics link to see the difference.
The 15" wheels had to large of a diameter lump in the middle to get the repro hub cap to snap on and stay put.
