Tim,
I have used a 50 amp 12v 1-wire alternator (that is now 18 years old) that my friend is still using on his 54 truck. It gets "excited" ("turn on point" or "cut in") at around 800 rpm. Once it starts charging, it continues to charge at all RPM until the engine is turned off.
A one-wire alternator has an internal voltage regulator.
To simplify wiring and to fake an original appearance, some people have taken the "guts" out of the voltage regulator and make the one connection inside the empty voltage regulator.
I have a 6v negative ground Powermaster one-wire alternator that I might try on my Suburban. They also make a 6v positive ground 1-wire alternator (perhaps of interest here to GMC owners).
