73untamed

i have a 1973 mustang and i have been trying to bring it back to life. well im having a bit of an electrical problem. when i run it during the day it does fine but at night when i run it with the lights on i dies on me. i have changed out the alternator, battery, regulator, checked for electrical leaks and it still doesnt want to run right. if someone could give me some pinters or at least put me in the right direction i would greatly appreciate it. your fellow mustang fan.