Plastic line is actually pretty strong and won't rust.  I would cut each
end off about an inch and re-use.  The return line is not under pressure
(unless your tank is too!).   Wilfy's kit (Van-again) is excellent.  I just
did my '85.  I haven't done the entire return line, but did have enough to
run from the regulator to the plastic line which goes back to the tank.
$omeday I will get around to all the tank ho$e$ and fitting$.

                                                   Jay