I have wasted huge amounts of time, material, and money trying to rehab old fuel tanks- - - -and I've rarely, if ever been really successful. I'd suggest that you try fabricating a tank from thick aluminum sheet metal,( 3/32" or so) folding the sides and ends into a box shape, and TIG welding the seams where the corners meet. Then fab a top with the appropriate relief area for the spare tire, and nest it over the box-shaped lower piece. Weld the perimeter where the trop and bottom nest together. There's no point in trying to duplicate those rounded shapes- - -just build a square-cornered box to fit the space that's available for the original tank. You might lose a few pints of capacity that way, but it won't leak or contaminate the fuel supply.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!