Master Cylinder for 4 wheel disc?

You probably should look at aftermarket MCs, like the MP aluminum MC, though it needs an adapter (2-bolt to 4-bolt). The reason is, the OEM disk/drum MCs have a residual pressure valve on the rear brake port -- this is for drum brakes only, so you don't want it. Or, find an MC from a later model truck -- Mopar moved the RPV to the rear wheel cylinders eventually (1980 or so?), so it isn't in the MC on those units. But I don't know what units interchange (if any). For 67-76 A-B-E Mopars, all the MCs are interchangeable physically -- disk, drum, power or not. But like I said, these would have the RPV.