MASTER CYLINDER: I JUST replaced the old one and we put a dual cylinder Ford Mustang 66-67 cylinder. Have no idea what the specs are as far as bore size, or which cylinder is the larger one...

More than likely your Mustang M/C is 7/8".

I had one for installation on my 32, but changed to a Torino M/C (same bore size) due to the Mustang outlets point toward the driveshaft and the Torino outlets point toward the inner drivers side (US) frame rail.

If your brake lines are clear of other components when the suspension travels and the wheels turn, you shouldn't have a problem.