I'm using 3/8" aluminum line, with those rubber-insulated clamps to hold it to the body. My fuel system isn't as straight-forward as yours, but basically from the rear axle forward, the line hugs the subframe where it's welded to the unibody, then along the SFC (photo), through the hole in the chassis that the original 5/16" line still runs (that's my return line), then up to the engine bay to the regulator. The aluminum line is really easy to work with, even in a double-flare. With the AN sleeves, you only have to single flare, which saves a lot of time / heart ache.
All the rubber clamp things are held on with SS self-tapping screws I got from Homie Depot or Lowe's, and that's where I got the clamps, too. I'm sure you could find clamps sized right for your flex line. If the clamps are spaced pretty close, you won't have any reall sag problems.
