I am very interested in watching this build. So please continue to update as you progress!
My father has a tank that almost looks identical to what you have here. Same color, just not as rusted... The reason we shied away was that it held gas. Now that was 20 some odd years ago and it's sat in the back yard empty since. I've heard never use anything that had stored petroleum because it will give off odd flavors... Is this really a concern if you wash it and burn it out?
I'll be interested in seeing how you build the fire box and attach your shelves...
Hope it works out for you!