As far as the Rally II's, those would not be original to the car. For 68, they were JC's - 14x6. The pic you posted is an HN - I'm pretty sure they would be from a 74 Ventura / GTO - I know, because I bought some from a friend's salvage yard way back in the 70's directly from a wreck. While they are not exactly the specs of a JC, my recollection was that they were about as close as you can get, as far as offset and backspace. Very miniscule difference. All 68 Firebirds had standard drum brakes, but you could get front discs as an option. The JC's fit both drum and optional disc for 68. It just could be that the very slight difference of the HN is the reason for not clearing the calipers - I suppose it could also be that these are not the original disc brakes? - maybe someone else can shed some light.

Nice looking car - enjoy the summer with the top down.