What I did was used a small drill and put a hole in the top of the fill tube. Then I put a punch through and made it bigger until a 1/8 pipe tap could be threaded in. I installed a hose barb. Then ran a braided AN hose up to the top of the quarter panel and strapped it down the tube through the fill tube rubber trunk seal through the floor. Same as all the 67 through 70 a bodies were done with a metal tube.
You could also look for a 67 through 70 fill tube and cap for a Dart
Here are pictures of what I did . The car was wrecked and no fuel spill or fire. The reason I didn't use a 70 duster style is because I already had two 71 caps painted.
Being your car is a scamp and if you want to replace the tube you will need to use a Dart tube. Duster will not fit
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Here is a picture of a 67-70 dart with factory vent tube
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