Your choices are either to use a '67-70 B body type disc brake master cylinder or the '71-'75 Mopar "one size fits all" master cylinder. The B body one has a 1-1/8" bore, the '71 up style has a 1-1/32" bore. The OEM master had a 1" bore. The 1-1/8" one, which can be purchased aftermarket as Raybestos MC36283 has a closer look to original as it has the "thermometer" style geometry on the front. The other one I mentioned is Raybestos MC36307. I've used both styles on my '70 and '71 PDB Challengers and both work, the 36283 has a harder pedal and the brakes are a little sensitive, so maybe the 36307 is a bit better functionally, but both work.
Disc brake masters should be painted semi gloss black.