I just talked to Lee at Tanks, Inc and learned a few things. First, my billet cap did not have the thick o-ring gasket, so my odor in the trunk is most likely due to my cap not being sealed. Tanks is sending me an O-ring which they say is very thick and heavy, rated for fuel service.

Second, their brass anti-rollover valve actually is designed to fit down into the tank as it has two check balls - one is phenolic and floats up to seal the vent with slosh with a full tank (not supposed to be topped off, but if it is...), and the second is stainless to drop down if the tank/valve becomes inverted. Mounting it extended above the tank is fine, but Tanks does not consider that to be necessary with their dual check ball design.

Lee says that it is normal and expected to have a slight gas odor from the vent in a closed garage, especially as the tank warms as ambient temperatures increase since our applications do not have the modern OEM vapor recovery systems and charcoal filters.

Hopefully the O-ring gets rid of a lot of my gas aroma, and I am still going to get a longer hose for a up & down loop seal.