going back to manual disc brakes from power

I can't speak for the 2 bolt master but the 4 bolt uses the bellows to keep dust out of the cyl bore of the master. As for keeping leaking master from the inside of the car... The missing paint on the drivers floorboard of my 67 dart says no! (Big sticky mess in and under the carpet)

As for the gasket that holds the bellows...

It is not reproduced. It serves 2 purposes,

1 is to secure the bellows, before 70 something there was a metal part that bolted to the master that kept the piston in the bore and attached the bellows. After 70 something the master had a snap ring to retain the piston so the gasket was introduced

2 is to act as a seal between the master and the body.