I used WD-40, but I'll bet PB Blaster, and a dozen other anti-corrosion concoctions would work just as well. I think just giving any of them a few days of soaking time is the key, before you try to back out any of the screws. Before you start, be prepared for dealing with stripped screw heads. Odds are pretty good if you have lots of corrosion, you'll lose the grip and strip the head on one or two of these screws in the process. See my project post here
Fuel Senosr Replacement, for a look at what I was dealing with.
I used micro-vise-grips to remove my stripped threads,
FishFactory dealt with the issue by turning the phillips head screws into slotted heads with a sawzall blade. You'll be extremely lucky if you get through the replacement project without stripping at least one screw. I put stainless screws back in.
If you intend to replace the whole unit, make sure you plan ahead and determine your fuel tank size, and order the correct sized replacement.
Keep us posted, and toss up some photos of your project when you're done. The more of these we get through, the more tricks we learn, and the better we are all prepared when we each need to start looking at fuel tank issues. I know that for me it was a big wake-up call regarding Pie Plate sealing, ventilation, and corrosion below decks.
John