The fill bolt is always tight. I jack the car up and get at it from underneath. Put your closed end 17mm on there and whack the wrench with a 3 lb sledge. Works every time. I posted in the other thread for what I'd do to get out the VSS.
Matt: They make a torch for Propane/Mapp which is for high temperature's. The one I have is on about a 3 foot hose. When you light it, it has a whirling sound to it, like a idling jet, sort of. If you have or can get one of those, they get hot enough to braze very thin metal with, but they can get the bolt hot enough to allow you to get it out. If you don't have one, they can be a very worthwhile tool.