Sounds like the alternator isn't charging. You're reading battery voltage.
Pull the plug on the alternator and check the white wire for 12 volts with the key on and zero with the key off.
This is the exciter wire that turns on the alternator. The blue wire should have 12 volts all the time with the battery connected.
If you don't get the readings then check the regulator plug for the same readings on the brown wire 12 on/ 0/off and the red wire for 12/12 volts.
If you get these readings then the alternator is most likely bad.
The MSD distributors don't like the voltage surges that the external regulator points generate. You would be better off with a SI or CS style alternator for more amperage at the lower RPMs. There are tons of conversion threads on the net and here in the forums if you're interested.