When out to the 51 and took some measurements.
It's a 51 Plymouth Cambridge I converted to 12v negative ground using a Mopar alternator and voltage regulator. Stock points ignition with Mopar ballast and 12v coil.
engine off, key in run I see 12.2V at the battery (low, should be 12.8v ish) and 6.2v at the coil +, all grounds taken at the battery negative terminal.
engine running, 12.8V at the battery (low, should be 13.8v ish, need to up the idle speed) and 8.8v at the coil +, all grounds taken at the battery negative terminal.
Pertronix says 8.0v minimum. You barely have the minimum for a 6v system. You aren't still running the ballast?
A
LINK to the Pertronix test pdf.
For some reason your voltage at the coil is way low.