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.

Depends on what's on the other side of the ballast.

Was that with the engine running?

From another thread where I posted the readings I got on my setup to help with a similar question.

One side of your ballast should be real close to the same voltage as at the battery +. If it's not you have issues upstream of the ballast. Once you get the new ballast installed check that.

The battery was at 13.2 and 5.5 to the ballast and 5.4 to the coil.

The problem I encountered is a no start issue. and no spark to the plugs. I though maybe the ballast went bad....time to check some other things. Note. that the end of the ballast was loose, and when I tried to tight it, it broke off. so I need to replace that.

That voltage is pretty close to correct. Points or electronic? If points you probably have the points stuck closed. If electronic check the ignition box and its ground.

In Run, engine off, the voltage of the current flowing into the reistor should be within a 1/2 Volt of the battery reading taken at the same time.
I don't believe the battery was at 13.2 Volts while the key was switch on.
12.8 Volts is about maximum for the stored power.
Readings above that are usually surface charge. Surface charge disapates very quickly when a load is placed across the battery (flip the lights on, dome light, etc).

Under very heavy load, the votlage will termporarily drop lower. For example, during starting the voltage will drop down to 10 Volts or a even less depending on battery condition.

Chrysler tech used 9.6 Volts at the coil as the rule of thumb when measuring at the coil during start.
And a maximum of 1/2 Volt drop between battery and run circuit with key in run and engine off.
p. 9, 1969 Ignition System Analysis
http://www.imperialclub.com/Repair/Lit/Master/259/Page09.htm

Might want to print that pamphlet. Not sure how much longer the Imperial Club website will be around.
(link to larger printable version of available at bottom of page)