I don't have documentation back to 71, but I've messed with later year fuel caps and if the OEM cap is functioning properly, I don't think it should be leaking like that.
Back to at least 72 there was a check valve in the cap that was supposed to let air in (to replace gasoline taken out to run the engine), but not let air back OUT.
Are you sure your check valve is working properly? If you've not ever messed around with one of the caps, it's the spring loaded metal disk on the inside center of the cap. If you pry that disk away from the cap, it opens an air passageway to the little radial bumps bent into the outside of the cap. The ridges that run perpendicular-ish to the grab handle?
On edit - Found some pics. here's the cap where you can see the air intake passage bulges:
Here's the air intake passage from the side:
Here's the check valve. I pried it up with a screwdriver:
And here's the air intake hole on the under side of the cap where the air intakes get to the edge:
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