Not going to give advice on your leak, don't know; HOWEVER:
In the USA, at various eras in the history of the automobile, metal fuel lines have been used on PRODUCTION cars, trucks, and tractors constructed from (in alphabetic order) aluminum, brass, copper, and steel.
Copper was still being used on PRODUCTION performance carbs built by Oldsmobile as late as 1966.
Copper, per se, is NOT your problem.
In the FWIW category, I started playing with cars in 1959, tractors earlier. To date, have had one metal fuel line failure - steel. And several rubber hose lines have failed.
Jon.
