If it's a good mechanical joint (wide mating surfaces) then you could probably use one of the harder grades of soft solder. Soft solder loses
its structural strength somewhere above 100C - 150C, but that's not a problem at the bottom of a fuel tank. Don't bother to tack-weld as
you'll cause distortion, just clamp it in place securely before soldering. Also, use the proper flux to match the solder in use. Don't
use electrical cored solder as its resin flux isn't much good on steel (it may work, but maybe it won't). One type of harder solder is
Comsol, available here amongst other places (but that place is expensive, so worth shopping around).
Background: I used to make live-steam models, and the harder grades of solder were used to make boilers in the past. It's strong stuff, used
properly.
Silver solder won't be much better than welding for distortion, as you have to heat the work up to red-hot to make it flow.
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