Was rolling over this morn when the thought pops into mind, if there's leak after welding the whole thing together, and fuel has been introed into tank, then there will be fuel in the gap between the welds, and the torch will light up the fuel!
Sometimes a seemingly great idea, making the tank back superstrong, can lead to unforeseen consequences.
So, just going go ahead with What I first did, make the back with fewer pieces, fewer welds, and no overlapping joints. Will hang this piece on wall as a conversation piece, I think I can pound out the two new pieces fairly soon using what I've learned.
I knew there was reason I never saw a lot of welds on the various tanks seen!