If the charging circuit is stock the voltage you are reading at idle is probably correct , mopar altenators of that era don't jump to 14ish volts till about 1400 rpm ... i think that is the rpm i have seen posted .

You should have something close to battery voltage at the coil when starting , you have something somewhere that is not maikng a good connection and has some resistance. How are your connectors from the column to the dash harness ?