I know I'll get flamed by the purists for saying this--but I've used flexible copper tubing for fuel lines on more than one of my trucks (both gas and diesel) and have yet to have any issues with them..no RUST at least!..replace the steel fuel line with copper properly and you'll never have to touch it again...
I know some GM repair manuals say to never use copper or aluminum tubing on things like fuel or tranny cooler lines,but why did so many vehicles back in the day have it factory, without any issues..?...
Yes,I dont use it on a EFI system or any brake lines,carbed engines only!--but I have used it with good results by using the correct fuel hose or rubber tranny cooler hose to connect the tubing at each end to the fittings,rather than flare it and screw it directly into the radiator or carb,etc,to avoid it from cracking from vibration..
There is a heavier walled copper "air brake tubing" available from Dorman that is able to withstand higher pressures and vibrations too..I've been tempted to use it on brakes more than once,but now they have that new copper-terne stuff in rolls thats worth the extra money if you live in the rist belt like me..