I had the same exact problem with my 67 mounting 4 piston calipers with 15" rally II's.....I ground down the casting where it was hitting - more than 1/10 - probably more like 1/2 to 3/4's but it was on a very thick part of the casting.....Also bought a couple of wheel spacers from JCwhitney that are 1/2 thick and voila - problem solved....
I think the set of calipers must have come from something other than a firebird - couldn't tell they came from FRANKS POntiac parts years ago and I forgot it was so long ago.

I agree with the last post that the 15" rally II's must have also been from a drum brake car because the offset from the base of where the lugnuts hit the disc to the spaceing behind the spokes was not very deep. 14" Rally II's came in two configurations one had a much deeper offset than the other - the depth of the spokes was very flat on the drum brake style and very deep on the ones that fit the discs.