Looking at McMaster-Carr's site, 0.063" thick 3003 aluminum in a 24x36" sheet is $33.22. It seems a lot of mistakes and rework is possible considering the cost of materials. For me, this would be an opportunity to practice and improve my mig (GMAW) welding aluminum skills. In for a penny, in for a pound. But, that is me.
FWIW, I have about four (or six?) books on sheet metal work and (eventually) intend to make some items. Your tank tank is a challenging item, but it looks like you have a good approach. From what I have read (all book learning), a rawhide mallot is a very useful tool for sheet metal forming.
The home build aircraft types have some online information about aluminum sheetmetal forming along with the tinmantech.com website (which seems mostly about welding). There is also a metalshapers.org website, but I have not found that especially useful. However, you may. A decent torch to anneal the aluminum after working and figuring out the annealing technique is also useful. I will be following your project with interest. Good luck!