I've found that for the cast iron OEM masters, the gaskets that are available are thinner and of a more 'plastic'-y material. When I was still running the OEM master on my '70 Dart I found this one from IH is just like the factory 'originals' in thickness and composition:
Disc/Drum Brake Master Cylinder Cap Gasket for 1974-80 Scout II, Terra or Traveler - IH Parts America . It solved the leak problem.
Often the same casting masters were used across a variety of vehicles and you can search for a gasket by the casting number.
