I'm going to preface this by saying I didn't read the entire thread, but at work I've had to clean up some pretty nasty tanks. First off if at any point either now or after cleaning if you see rust replace the tank. I like to pour in enough carb dip to cover the nastiness if the nastiness is up the side of the tank then slosh it around occasionally. By the next day the nastiness is dissolved, dump it out, rinse with water, then with brake clean or carb clean or even the $.99 fuel drier. Reinstall and drive. Has worked great for me and requires little effort. although in a case where I new tank is $70 I'd advise my customer to replace the tank because even a small labor charge for clean up and the supplies would exceed that. Even though the OP is not paying labor to himself, his time is still valuable.