When you say they are not a factory cap, do you mean they are after market,,or did not come from the factory on a Z28.
I want to use them on a ZQ8 wheel like your first pic. So I guess I'm good. Actually, I'm trying to put some ZQ8 wheel's on my full size truck. I have found a place that makes replica wheels. I'm trying to see if I can get them with the correct bolt pattern. Then I just need to make the center caps work. I was going to get these to experiment with.