12" is the largest I have ever seen fit a 15" wheel and it takes an aftermarket mag to do that
The problem with steel wheels is mostly the shallowness of the wheel, they can hit the face of the caliper
Here's my options:
The old tried and true Chevelle based rotor kit, will 100% clear your wheels, 11" rotor big GM calipers but moves wheel outward 3/4". Never was an issue until people started lowering their cars. Bracket set bolts to stock spindles
Zero offset kit designed to clear 14" wheels. Will fit your wheels 100%. Uses 10.5" rotors with a low profile GM caliper
My designed zero offset 11.2" rotors big GM caliper seperate hub/rotor bolts to stock spindles
might not fit your wheels. Will not fit 55/56 15" wheels, will fit 63 newer Corvette probably fit most any newer 15" wheel
fitment issue is due to shallow wheels hitting face of caliper