7mm 'ignition wrench' on a 2006 SC filler clamp
Here is my experience on a 2006 9-5 fuel pump replacement, quoted from my post on another forum. I certainly had the same problem that you are facing. I remember that for re-installation I used a 7mm 1/4 inch drive socket and a "flex drive" to get that clamp tight again.
"WIS says remove filler pipe retaining bolt – it doesn't say why, but it is necessary to move the filler pipe enough to undo the filler hose from the tank. On my car this is an external Torx bolt, and was hard to remove (did not want to break it!). WIS says to remove filler pipe hose and "bleed tube" from the tank before lowering it. The bleed tube needed the GM connector tool again. WIS does not say how obstructed this area is. On my car the filler obviously went in before they installed the rear subframe, because the "airplane" screw clamp on the filler pipe hose was installed with the screw at the bottom where I could not see it (there is almost no room). I thought this might not be possible, but I used a 7mm open end ignition wrench and loosened the clamp."
Hope this gives you some encouragement that it can be done.