oh christ, don't use tm's tech flux for a tank
3003 is fine for a tank, use 14ga (.063), not 16, that's too thin. 5052 work hardens too quickly to be used much in forming complex shapes like tanks, fine for bending etc. 3003 also has the best overall corrosion resistance.
tig is probably best for someone who didn't learn to weld in the 30's, gas welding is mostly a lost art on aluminum and tig will do just as good a weld in trained hands, gas welding on aluminum like that and getting a strong consistent weld is very hard (I've been building tanks for 6 years now and still prefer tig because I trust it more).
secondly, have you done any fabrication work with aluminum yet? a tank is not the best first project. maybe start with a seat that's made in a few pieces so you get familiar with joining multiple pieces to make a single more complex shape.
thirdly, what tools do you have at your disposal? how much do you know about things like how to attach filler bungs and petcock bungs? what type joint were you planning on using to join the top and bottom?
if it's something you're seriously pursuing, I can give you some input if you want