thanks for the warning, fiid
i did something like this once before on my old morris minor (not the tank) -- i think just the fuel rail piece would be okay as long as it got a thorough wash, dry, wash, dry, and then a test pass with a torch.
i have a friend (mentor, really) who used to weld fuel tanks. crazy motherf-cker. he would top off the tank COMPLETELY and then weld. the theory was that no O2, no explosion. can't believe he's still around.
of course, gas tanks need to be steam cleaned before welding. or better yet, taken to a professional