Hub vs Lug Centric Wheel fit

[NAPA 68]

I have searched the archives for vibration. I did see one post that spoke of wheel fitment issues (thus causing a vibration) for certain wheels. Here is the problem I am trying to solve. On my 67 Nova, I have GM steel wheels that I have checked for being bent, runout, etc. These appear to be fine. I am on my second set of tires (Bridgstones.......not just cheap tires) and have had everything Road Force balanced. I am still getting a fair vibration in the front end from 70mph on up. Due to the fact I switched to a rack and pinion last year only made the problem more apparent. Worse yet, it is inconsistent. sometimes it is worse that others. That leads me to the wheel is moving around on the rotor. The car does have a disc conversion of unknown origin (on the car when I bought it)

My question is, how tight should the wheel fit the hub of the rotor? When I first purchased the car, it had some aftermarket ralley wheels that I not recall having an issue like this. Buying another set of wheels is no problem. I would just like to diagnose the problem rather than shoot parts at it.

Thanks,

Tim