Start by reading this article:
http://www.madelectrical.com/electricaltech/amp-gauges.shtml
more where this came from
http://www.madelectrical.com/electrical-tech.shtml
The next thing is, "the alternator"
The older "roundback" alternators do not put out nearly as well at lower RPM as the newer "squareback" type. You might consider asking for a 45-55 or so and make SURE it is a squareback. This will have TWO field terminals, so to hook this to your old 69/ earlier regulator, you'll need to ground either one of the two field connectors
You can also, of course, add just one wire, and convert to the 70/ later regulator.
example of a "squareback"
The "usual stuff"
You might have one or more bad diodes, turning your 35A into a 15A, or your 45A into a 20A, etc. So test the alternator for output. Read the shop manual
Someone might have put a larger pulley on the alternator slowing it down
The pulley might be glazed, slipping, or the belt might be worn/ loose
Try a few tests
1---Check for voltage drop in the ignition circuit which also supplies the regulator. Turn the key to "run" engine off. Hook your meter to the IGN terminal of the regulator, and to the battery stud on the regulator. With the meter set to low DC volts, you should read a VERY low reading the LOWER the BETTER. Over .3V (three tenths of a volt) shows a voltage drop problem in the harness. This will actually cause OVER charging at higher RPM
2--check the regulator ground. To do that, get engine running at an RPM to simulate "medium cruise" As above, low DC volts, stick one probe on battery NEG post, and the other directly onto the regulator mounting flange. Again, you hope to see almost nothing for a reading
3--With the engine still running, check battery charging voltage. With the engine/ regulator warm, you should see 13.8--14.2, and in no case more than 14.5, nor less than 13.5
4--Check for voltage drop in the alternator output lead. Clip one voltmeter lead to the alternator output stud. Clip the other lead to the starter relay battery stud. Now turn on some loads, lights, heater, 4x flasher. You hope to read less than 1/2 volt. If you get a reading approaching 1 volt, you have voltage drop problems in the charging wire. I defer to the MAD article posted above.
Your top suspects for voltage drop problems in these old girls are A--poor connections in the bulkhead connector, not only the charging line, but the headlights and ignition circuit. B--poor connections at the ignition switch connector or the switch itself. C--Poor connections in the headlight switch or the switch itself, as well as same at the dimmer switch, and again, bulkhead. Also check the headlight grounds out at the headlights as well as condition of the headlight connectors.