This is from JBMopars Duster build up from a few years back. This car has so much cool stuff happening, hope he doesn't mind me posting these. He is an excellent fabricator. Have not seen him post here for a few years.

Anyway, he put this bar between the rear frame rails with a tube sticking out rearward through the rear bumper. The license plate flips down on this car. Duster filler tube is on the left rear quarter, Dusters don't normally have flip down license brackets. (at least mine doesn't) Pretty slick!

IIRC, the fill tube on an early B body is behind the flip-down license plate bracket. Maybe you could figure something out.







And then...