There is a gas tank coating around - I don't remember the name of it, but the antique tractor Guys use it. I have seen it on their sites. Its for plugging holes in tanks. If that neck is all your worried about, once its clean and installed, it won't rust more, in an active gas tank. The tank liner coating label is good info though. It talks about how to get rid of rust and other chemicals to use to clean stuff.
One trick I have used to clean stuff like that is sort of low tech. Get a piece of 1/2" wood dowel & use a hack saw to saw a slit in one end about two inches long. Then cut a long strip of course sand paper/emery cloth, and put it in the slit. Install in an electric drill, and run it inside the neck. Some cylinder honing tools might work too.