Vented Gas Cap

Do NOT use a vented cap. None of these cars used vented caps except the ones 70/ later with the evap emissions (carbon can) system, and these were not truly "vented." They were called "pressure vacuum" caps and had what amounts to pressure relief valves.

The right way to do this is exactly how the factory did it up through 69----a vent fitting up high on the filler, which goes up even higher in the rear quarter, makes an upside down U and goes back down and ends pointing down in the rear frame rail.

All a vented cap will get you is............fuel all over the side of the car

What is the one in the photo and why do you have to modify it?