what master cylinder for disk/disk set up?

Larger rotor size has nothing to do with master cylinder size requirements. Using the larger late B body rotors in place of the smaller ones has no effect on other brake system components. Inlarging front caliper bore diameter (73 to 75 A bodies used a 2.60" diameter piston bore, all other A or B body single piston calipers were 2.75" diameter) may or may not require a different bore master cylinder, replacing rear wheel cylinders with calipers when changing out to rear discs most probably will.

As master cylinder bore diameter increases, the amount of travel needed to displace a given amount fluid decreases resulting in shorter pedal travel. As the bore diameter decreases, the piston stroke must increase to displace the same amount of fluid. Use of a master cylinder with too small of a bore results in too much pedal travel, and the pedal bottoms out on the floor before adequate pressure is generated to stop the rotor from turning. With any given amount of wheel cylinder/caliper bore surface area, inlarging the master cylinder bore diameter increases the amount of foot pressure required to move the master cylinder piston. High pedal pressures are why large bore master cylinders are used with power boosters. The short pedal travel resulting from the combination of a low effort power booster and large master cylinder diameter could also explain why the brake pedal action in older power drum brakes systems seemed more like an on off switch.

Drum brake cylinders do not require the volume of fluid that disc brake calipers do. You must use a master cylinder with adequate reservoir capacity for the size calipers you are using. Most disc/drum or drum/drum master cylinders are marginal in the capacity of the reservoirs intended to service the drum brake cylinders.

Residual check valves haven't been used in most or all OEM drum brake master cylinders since the advent of the metal wheel cylinder seal expander (late 70s).

If you are going to run without a power booster, depending on pedal ratio, a 15/16" or 1" bore is usually about right.