The way you have built your tank, you could use Proseal and sealing rivets. a lot of wet wings are done that way... Mooneys for one. Then there would be no welding and probably just as lite. Just a thought. There are different brands available. kind of messy but it works.
If you are going to weld it Don't use AD rivets, they are a 2117 alloy which are not weldable. Use soft rivets they are weldable and are the same alloy you are using. BTW you do nice work.
As some sage once said, "A man's got to know his limitations." I can weld thick aluminum pretty well. I can run a nice bead if it's laying flat with this .040. I can weld over pop and conventional rivets. I can fill a gap as big as 3/16".However, I'm not happy with what I have. I'll just take it to my old friend Scott, and have him do it. I would probably waste enough argon practicing to pay him..