[Lorenfb]

"also, richard explained that most alternators are 'underrated' by maybe 5-10 amps, so that a new 55 was probably 60-65 amps if bench tested when new."

Not true! That alternator is even marginal at 55 amps. There's NO way of
getting more amps without a larger stator (which won't fit) & higher capacity
plates diodes. The alt. tester used needs to be calibrated & is probably off
by 20 amps TOO high. The other factor is the voltage measured at the load
current, e.g. probably 50-55 amps @ 14V, 12V @ 60 amps, 8V @ 65 amps.

The key for any alternator is not only the peak amps but the
sustained amps under continuous load AND high engine temps.

Bottom line: Your rebuilt is basically the same as the OEM original
which probably developed a lower output over time because of;
worn brushes/slip rings, weak diodes, bad diode wiring. & etc.