The main problem is the stock filler tube has no baffle to stop gas from being forced up the tube on sharp RH turns. This is exacerbated with a full(er) tank. Check that the rubber seal on the cap is in good shape and is still pliable so it seals to the neck's lip. The cap is vented to allow air into the tank as the level drops so even if you have a perfect seal on the cap it will still leak out the vent. Another area to check is the rubber seal at the tank/filler neck juncture. This is not the flat rubber seal on the trunk floor the fill tube passes thru. This is just a weather seal for the sheet metal opening. The tank seal is a doughnut shaped sleeve that seals the gap between the filler neck and the tank. Rubber hardens and can develop cracks or even split over time. If gas leaks here it drools down the side of the tank and right onto the exhaust pipe.
There are baffled tubes available, Layson's is one source, or you can make one yourself but it will have to be welded in. New caps are also available but are ridiculously expensive last time I looked. If you replace it be sure you get an original replacement for the 65. I've tried a bunch of different caps and nothing works better. Replace the doughnut if you pull the tank. It is going to be really stuck to the tube/tank if its still original (its been there 50 years!). A leak here should be readily obvious by wetness on the drivers side of the tank.
