WD40 does not work well as a penetrating oil- it was designed to displace water. Get some PB Blaster or Kroil and let it soak for a day or two and make several applications. You will help it work if you brush off accumulated dirt and rust from the application site.

I have, at times, resorted to an impact gun to break these fittings loose. At any rate make sure you are using a box end rather than an open end wrench (and a 6 point box end is even better). I have found it easiest to go to the extra work of removing the whole assembly to the bench for easier working conditions and space. Be prepared to plug the inlet line as gasoline flows out due to gravity. Always be prepared for the worst case when dealing with potentially dangerous activities like fuel spills and leaks.

Hopefully you got new copper washers with the new fuel filter. Always check as soon as you start up the car for leaks from a newly installed filter. I have found it counter productive to try to hold the filter by the hex shaped form at the one end. The aluminum case of the filter is easily punctured with the slip of the wrench. I use a very BIG pair of adjustable pliers to hold the filter by it's built up seam.

Randy