Here's a coincidence: look at my avatar, then look at my album, then look up the thread "Scored a Higgins". If it has no cracks, I'd say you have a rather solid and good looking vintage bike frame. Mine was very well suited to motorizing. You seem to have the same.
The tank I'm using now is a torpedo style tank that I made from an aluminum softball bat. But because it's rather more slender and smaller than I'd hoped for, I'm am trying to improve my technique with using Alumiweld so that I can make a much larger in-frame tank entirely from scratch. When I feel ready to begin, I will post my methods and pics under the old thread I mentioned - "Scored a Higgins".
But if you want to see a master at work, talk to Tinsmith (Dan) and Silverbear. They are making one from copper for SB's Hiawatha. It's looking good.