Why does a rusty stand make you worry about the seams? If the tank is passing it's leak test and the seams visually look like they are in decent shape, I wouldn't touch them.
Maybe the prior home of that tank wasn't ventilated properly?
I have resealed a number of tanks up to 90gal, but I never removed the silicone between the end panels. I just removed all I could with a razor blade from the inside of the tank and resiliconed. I'm not sure I would trust that on that big of a tank. Probably the only way to do it properly would be to disassemble it completely and rebuild it...not the undertaking I would want to embark on!
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