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.