[RichardJ]

The real difference between a MC for power brakes and a MC for manual brakes is the hole on the end of the piston. Shallow hole is for power booster, deep hole is for manual brake rod.
CPP for any given bore diameter, sells the same MC and includes an adapter pin insert.
Piston bore diameter differences won't stop brakes from working. A larger bore requires greater foot pressure, so is generally used with a power booster. Smaller diameters require less foot pressure and are used with manual brakes.
The pin on the front of the booster is adjustable and must be checked for proper length before mounted to MC. There are checking tools made, but if you are careful you can check by making measurements.
Using the larger reservoir for disc calipers is done so that it doesn't run out of fluid before the disc pads wear out. You'll have brakes with the small reservoir, but you better keep a close eye on the fluid level.
The factory used a COMBINATION VALVE.
One end of the COMBINATION VALVE has a proportioning valve and is plumbed to the rear drums.
The other end of the COMBINATION VALVE has a Metering Valve and is plumbed to the front disc calipers. The Metering Valve is identified by a pin that must be pulled out or pushed in, depending on the type, when bleeding.
If you have the brass type combo valve the Metering valve pin is pulled out when bleeding for most of them.