Steve, without going thru the numbers, I have to believe that the difference in weight of 0.040 SS vs 0.090 aluminum tank has got to be less than a couple of burgers, 1 good piss, or a 1/4 gallon of fuel. Maybe I'm off by a factor of 3. Still not something I would be concerned about.
You could always make the tank just a bit smaller to offset the added weight? But if E85 is a your goal, I'd say do it once in SS and move on. But it is your dream and your car. So my thoughts are worth what you paid for them.
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Chuck.
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