Any Master Cylinder built and sold for use with Disc Brakes ought to not have RPCV in its hydraulic outlet leading to the disc calipers. I have always felt it was best to purchase a MC made for Disc Brake systems as the cylinder bore diameter can be different as compared to MC units designed for wheel cylinder (drum brake) use. For something involving retrofitting disc to replace drums I would likely get a full set of parts (calipers, MC, PB Booster) from a wrecking yard where the parts came off a 71-73 Mustang, or (better yet) get the parts from Wilwood. Were I to be getting my parts from Wilwood I would prefer to call and talk to someone about the parts I ought to be getting.
I would not consider doing a disc brake conversion without a power brake booster. That is because disc brake systems do not have the same kind of "self-energizing" design as a drum brake system. With the drum brake system the top anchor pin is what absorbs all the braking pressure as the "floating wedge" action of the brake shoes/linings are essentially jammed outward against the rotating mass of the drums, which essentially aids in forcing the friction material of the linings against the drum surface. The disc brake system simply uses hydraulic pressure to squeeze the pad friction material against the rotating disc, and no self energizing action is present. I once had a car with non-power disc brakes. The brakes worked well enough, as it was a smaller car. But, I had to exert a lot of manual pressure onto the Master Cylinder to get the brakes to perform properly. Never again will I have a disc brake car without a power booster. When it comes to brake boosters on these Vintage Mustangs (and Cougars), my instinct would be to call WCCC and seek their recommendations. In fact I might end up doing all the parts purchases from or through them. They have a terrific reputation
An alternative approach if the drum and disc MC units do have the same bore size is to remove the brass seat n the MC outlet that will be feeding the disc calipers, pull out the RPCV, then install a new brass seat. I would be prone to reusing the original brass seat as it will likely become distorted when being removed using a machine screw in it to all it to be pried out of its position.