I had an auxillary fuel tank on my Sonoma in the back, bolted to the box, and ran 2 copper lines from it to the change-over valve, about 10'. I ran WVO and diesel through it, and it was never a problem. I removed the lines recently, and they are still fine. This line ran under the box, along the frame, crossed under the transmission to the other side, then back uo to the engine. It was there about 1½ years.
"Work hardenening" comes from flexing and that can come from vibration. If you can support the lines from movement, I believe the chances of them failing will be greatly reduced to non-existant.
Just my 2¢.
