Cardinal, you should definitely be getting more juice with your loads on, unless your idle is really low. When you do the test, you should always rev the engine up to at least 1500 RPM to see the difference between idle RPM alt output, and full output. While most alternators produce full output at around 2500 RPM, in the real world there is only a minute difference at 1500 RPM.
There is no practical way to test the diodes without dismantling the alternator, however an alt with one diode failed will put out approximately 2/3 current, and with two out will put out about 1/3 current.
If you have a second car, most all parts stores will test your alternator output for you if you bring it in to them.
Do not overlook your wiring and connectors!
Both your 60 and Matt's 80 will have the solid state regulator integrated into the alternator; when they fail, they will usually fail catastrophically, unlike the electromechanic external regulators in the older models. However it is certainly possible that the regulator may be unable to pass sufficient current.
It should go without saying that your alternator belt needs to be properly tensioned. The easiest way to know that the belt is too loose is the squeal right after start up. Overtightening will wear out the bushings prematurely.