I'm thinking this probably isn't a voltage problem. Car electrical system vary from 12 to 15 volts when operating, but volts high enough to kill the headlights would be killing other things as well.

On GM stuff, the headlights are protected by an "automatic resetting circuit breaker". I'm betting this breaker is old and corroded.

Have you tried an ohm measurement of the low beam filiment after removing the bulb? See if it's really dead, or you're just replacing bulbs while the breaker resets...

Also, connect a volt meter to the headlight socket and rev the motor. I'll bet the volts don't go over 15, but if they do, then it's the voltage regulator.