Master Cylinder bore size?

I have a 1970 Dart Swinger 340 with front disc brakes and rear drum. i cannot say that the car was original with front disc. The question I have is "can someone tell me what the bore size would be on the existing master cylinder" without having to remove the existing and measuring before ordering a replacement? The existing master cylinder chambers look to be the same size, if that helps.
Need a little more info.
Are the brakes the year-correct 4 piston Kelsey-Hayes discs with a 4" bolt circle? Or are they the later '73 and up single piston caliper with a 4.5" bolt circle rotor?
Is this a power brake car, or manual brakes?