The two master cylinders are NOT interchangeable !!
The size of the reservoir is larger for disc because as the disc pads wear the caliper piston does not return all the way back into the caliper. No return springs. The brake fluid stays in the caliper.
The real difference in the two master cylinders is the check valves in the master cylinder. Drum brakes, with the check valve in the master cylinder all ways have a little pressure in the lines and wheel cylinders to keep the brake shoes cocked.
Disc brakes have no pressure in the lines or calipers so they can just relax.
If interested I have a NEW power brake set up complete, that should fit your car. Bolt on. For drum brakes.
