STOP! The very FIRST thing to concern yourself with is WHERE did you get the disks? If you are saying that you installed front spindles and disks from a 1980s G-body, DO NOT drive the car. The G-body spindles are not a bolt-in replacement for the 64-70 A-body cars. The ball joint studs have a different taper and will not seat properly in the spindles. THIS IS NOT SAFE TO DRIVE.
Did I mention that this is not safe?
Once you get the proper spindles and disks for an A-body, then simply get the correct master cylinder for an A-body with manual disk brakes - it was a factory option. Problem solved.