The strategy of a problem like this in general is
1--First define the problem. Your voltage readings have done that--the system is not charging, or very very little.
2--Determine the general area of the problem. That is, bad alternator, bad regulator, a wiring problem or bad connection
First, let's be sure what you have. Since you have an older engine, someone MAY have used the old alternator, and we need to determine if you have the 69/ earlier system, or the 70/ later system.
The 69/ earlier system has just one "push on" small field wire connection on the alternator
The 70/ later system has two "push on" connectors, that is, two field connections.
The 69/ earlier regulator has one screw terminal and one push on terminal
The 70/ later regulator has a rubber two terminal connector.
I'm going to assume that you have a proper 70/ later system, post back if that is not the case.
First, a quick couple of checks:
As previously mentioned, the regulator may have lost ground. Dismount the regulator, make sure the bolts were tight (not stripped) and clean and scrape around the bolt holes on the firewall and regulator. Use star lock washers. If there is any question, add a no 10 or larger ground from one regulator mount bolt to a certain ground point. The regulator MUST be grounded "same as" battery voltage
Next, Find the green field wire and disconnect it from the alternator. Take a clip lead and connect to the field terminal on the alternator you just now exposed. With the key in "run" hook a voltmeter to the OTHER (blue) field connector without disconnecting the wire, that is, "back probe" the connector. You should read "same as" battery voltage
If possible, with the engine shaded from the sun, connect the clip lead to ground. You should have one end to ground, the other hooked to the alternator field where you disconnected the green wire. Connect / disconnect this and watch for a small spark. Also check your meter. The voltage may drop a SMALL amount when you ground the clip, but not much.
This shows that the field circuit is getting power, and that the brushes and field are conducting current.
Now start the engine and watch the ammeter. When you bring up RPM the ammeter should show a hard charge, hard to the right. Turning on lights, heater, and with the engine at fast idle, should keep the needle to right of center at least some what.
If this does not happen,
with the engine running fast, Check battery voltage and voltage at the large alternator stud, with your clip lead stilll grounded. If the voltage at both points is close to the same, below 13, it is not charging
If voltage at the alternator is quite high, much above 14, but 12.5 or less at the battery, then you have a wiring problem in the charging circuit.
If these tests show no charge, replace the alternator
Post back with results.