If by "dually" you are referring to the rear axle configuration; disregard that aspect of the job - the fuel lines don't go much past the rear cab wall:
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The routing is along the back of the cab, a 90 degree turn toward the front, inside the rail for a few inches, up over the top of the rail just forward of the RH rear cab mount and then outside the rail for the rest of the way. Below are some images showing the way they are routed. The frame-off photos are of a crewcab (with dual tanks) - that is why they are so far back. With a standard cab, they would start one cross member forward of the way they are shown - see the yellow tracer in the 1st image. The rest of the images show a standard cab. I'll let you gauge the difficulty of the job:
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To eliminate the possibility of losing a line to rust you could fabricate your lines out of stainless steel (or copper-nickel) tubing.