If you like the look and want one on the bike, they make much better luggage.
If you're determined to have an ammo can gas tank though, I'd suggest looking for an already-made fuel cell in a convenient size and dressing it up to look like an ammo can, or perhaps getting a plastic one that will fit inside of an ammo can, rather than trying to weld-up an actual ammo can for use as a tank. By the time you weld the seams, weld the top on, and install the fittings, it won't look so much like an ammo can anyway.
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