stock 66 charging system?

Because it probably has the 36 amp alternator, and even at higher rpm will only put out 36 amps, significantly less at lower rpm! A change in your charging system is recommended! You can run the newer style voltage regulator with a simple change of the plug, upgrade to the squareback alternator to a 60amp unit, and eliminate the dim lights and weak output! To change the alternator is an easy job as well, with adding another field wire for the 2nd field terminal and splicing it into the blue wire on the ballast resistor the only additional work needed!