Up until 1970 nobody was making many 15 inch tyres!
I remember being in LA about 15 years ago and I saw a set of 32 inchers on a 4x4.
Things change over the years and guys/gals who stick to 14 inchers are probably are just trying to be true to 60's muscle!
If you are going to fit 1970 single piston disc brakes/calipers then its gotta be a 15 incher I'm afraid...
The old kelsey four pots are a 'mare' at the best of times...
Gotta keep it realistic, and make life easy for yourself.