I've heard of Baer kits hitting 16" wheels, which is why I had to go with 17"s on my 67.
If you can still mount the wheel and turn it by hand, then I'd imagine that you could remove enough metal from the calipers to give yourself clearance.
Is it making contact across the entire outside surface of the caliper, or only at a high spot, such as a corner?
Can you also use different wheels? You said you have to run "stock 14" wheels", but perhaps there are other "stock 14" wheels" for a Mustang which might give you better clearance, or at least have enough metal inside the wheel to machine some metal from the wheel, and also some from the caliper, giving you even more clearance than by removing it from the caliper alone.
If you can still mount the wheel and turn it by hand, then I'd imagine that you could remove enough metal from the calipers to give yourself clearance.
Is it making contact across the entire outside surface of the caliper, or only at a high spot, such as a corner?
Can you also use different wheels? You said you have to run "stock 14" wheels", but perhaps there are other "stock 14" wheels" for a Mustang which might give you better clearance, or at least have enough metal inside the wheel to machine some metal from the wheel, and also some from the caliper, giving you even more clearance than by removing it from the caliper alone.