Frankly what I would do is duplicate the early (69/ earlier) style vent. It was simple, and it WORKED. Here is a diagram right out of one of the service manuals. It has a 1/4" tube welded into near the top end of the filler tube, a tube goes high up against the quarter panel top, and down along the filler tube, through the trunk floor gasket, and the tube then just open ends into the rear frame rail.
A relevent thread
Fuel cap and tank venting
and the early vent system
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