Will a b/e-fuel tank fit with a modified a-body pan?

You might be able to use a Challenger tank, but you would have to swap the fill tube to the passenger side. Cuda tanks have a rear fill tube, but the tube would end up in the tail panel of your Duster. A friend of mine used a Mustang tank in a Duster. I fit well, but the gas cap being in the middle of the tail panel just looked odd. I had a custom aluminum tank with baffles made for my 70 Duster when I was gonna swap to a 5.7 Hemi. I cut out the spare tire well and was going to make the trunk floor flat and bolt it up with longer straps hooked to bolts in the reinforced rear cross member. Sold the Hemi and never put the tank in! It's too heavy to ship, or I'd sell it. It's a 25 gallon tank. You might try to find an old 68-70 Roadrunner tank to mock up and see if that would work. Might be too long? Don't know? Good luck with it.