What type of solder and flux did you use? You'll need an acid flux or acid core solder. In my experience, the issue with failed solder joints is not getting the parts hot enough for the solder to flow. Also, if the crack is more than a hairline, the solder won't flow into it via capillary action, so if it's an open crack, you'll likely have trouble getting it to seal. Replace tank in that case.
I'll offer another approach. Buy a can of Red Kote from the auto parts store (I found it at O'Reilly's) and coat the interior of the tank with it. I used it on a 72 tank that was riddled with pin hole leaks. Worked great and is still holding up 4 years later. It should seal a fine crack, especially if you put a piece of tape on the outside.
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