Master Cylinder Studs Size And Thread Count?

‘69 Dart Hard Top, manual drum brakes. Looking for the size and thread count for the 4 studs used to mount the master cylinder to the firewall. Mine are in place but have been painted over and the threads need chasing. Couldn't find anything in the service manual. I’ve tried a couple of nuts but they would not start and I don’t want to booger these things up more than they are now. Thank you in advance. All help/advice appreciated.