Re: help with fuel line fitting
You may be slightly hosed.....let the PB Blaster (good stuff!) soak in for a while, and try again. Unfortunately if it is bummed up badly, your only choice may be to use vice grips and get the thing off. If its rusted badly and won't easily come apart, you'll probably find that if the threads break loose, the threaded part will be rusted to the actual fuel line, and it'll start to twist the line as you loosen it.
If I were in your shoes, I'd loosen it however you can, and replace the line between the tank and the filter. Unfortunately, the tough part is going to be attaching to the sending unit, because the male fitting there is the same as the fitting going into the fuel filter. You need to find a female hose end that will fit this fitting.
http://www.raceprovenmotors.com/cart/images/DSC00526.JPG
Once you find that, get the hose end, 6' of -8AN braided hose, a -8AN straight hose end, and the above fitting, and you'll be able to replace that whole section. I've never felt comfortable using rubber line and clamps on fuel injected systems, or compresson fittings to bite onto existing metal lines. When I used to work at NAPA, they sold the above fitting and a 6" or 8" section of flared 3/8" line, specifically for fixing situations like you have. It hung on one of the spinning trees in the showroom. It would screw into the GM filter, and gave you steel line to bite onto however you decided to do it. I don't know if they still carry such a thing or not; that was 8 years ago.