[Blue_Heron]

What you have is called crevice corrosion. Aluminum's resistance to corrosion comes from a thin layer of oxidation that forms on the surface of the metal and acts as a protective layer against further oxidation. But it has to be able to breathe to work. In a tight space where water can be trapped against the aluminum, you get the type of pitting you've got.

I wouldn't trash the bottom of the tank unless you've got large holes or the pitting is extensive. If you've got someone who can weld it for you, clean out all the pitting with a wire brush and let him weld over the holes and fill the pitting. Just make sure you get all of it. And before you start the process, make sure you clean the tank thoroughly to get all fuel and fuel vapor out. When I did mine, I emptied all the fuel, filled it to the top with water and a healthy dose of dawn dish washing liquid, and then filled it and drained it twice with clean water. By filling and emptying the tank three times, you can be sure you've purged all of the fuel vapor.

You will probably want to coat the tank before reinstalling it to protect against future corrosion. There are several options. I use epoxy resin and glass cloth, but there are more economical ways to do it.
Dave