Hi Angella, I used S.S. lines on my 67 Gt500 from the Right stuff and at least half of the fittings leaked.
The problem is with the tubing flare. They are deffective by being off-center, incomplete, or overflared causing a crack. It you take one apart that is not leaking and compare it to one that is, the problem will be apparent.
I got them to replace a couple small lines. The other lines were to much of a pain to deal with so I fixed them myself. It is not that hard.
You may have to cut the existing flare off and make a new one. If you only cut just enough off it should not be a problem. Some just are not flared enough. Directions are usually supplied with the double-flare tools.
A tubing cutter and double-flare tool are each available from sears or harbor freight, northern tools, pep boys.
The tube nut only needs to be tightened to 10-15lb-ft, so if you feel the need to keep tightening that fitting - there is something deffinately wrong.
Just remember to take your time - something that was not done the 1st time!