The sealed beam headlights that came on a classic Mustang were standard equipment back in the day. At night it was fairly easy to out-drive your headlights, meaning the distance required to stop was greater than the distance that you could see. It's sort of going on faith if you will. In an early Mustang, a switch over to the brighter halogen sealed beams can be a big help. However, if your car has the OE headlight switch, the circuit breaker built into the switch can begin to overload and the headlights will flicker or go out altogether. Replacement switches with a higher rated circuit breaker are available and they will allow you to run somewhat brighter beams, but you're still limited by the factory wiring when it comes to running more powerful headlights.