If you want to be an environmental vandal, just run a breather from the tank , up the D pillar to the outside of the car. You only need a pinhole size opening.
The vent has to be far enough above the top of the tank so that it cannot siphon, if say you are stopped on an extreme angle. I had an auxiliary tank inside the RH side of the rear load area in a 77 , and I recall being on a trip stopped on a hill and watching the fuel siphon out.
You can add an inline filter as a small reservoir which will cut down vapours.
BTW the earlier model RRCs had a small square tank high up in the RH mudguard to act as a buffer reservoir to account for angles.
Regards Philip A
