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On my 67 250 just had a similar problem, although it was more like the headlights were "pulsing" on and off after being on for about a minute.
Ended up finding the harness on the pass side had slipped under the battery, and that caused the wires to be worn through and causing an intermittent short.
Cleaned them up, resealed then individually and shrinkwrapped them, problem solved.
I've also had wiring problems around the battery, was related to a trailer wiring kit that I recently un-installed.
Glad you found the problem, they can be dastardly hard to find!