Be sure to use genuine Neoprene fuel hose, NOT rubber hose.

Replace it from way back near the tank (where it loops up over the rear axle) to as far forward as you can get. Tie wrap the Neoprene hose alongside the original pipe for support.

The chances are that if you have one pinhole, the rest of the steel pipe is like Swiss cheese and will crack, crumble, break as you try to install the hose. And if it doesn't break immediately, another pinhole will rust through this winter just ahead of your patch. Do the whole thing, do it once.

"...a risk that the downstream pipe will act as a siphon and empty the tank..."

No. The return line empties into the top of the tank and is not submerged beneath the surface of the fuel. Even so, expect a cup, or so, of fuel to dribble when you cut the line. Have a pan handy.
--
Don Foster (near Cape Cod, MA)