Fuel Tank Question

I have a 1967 Chevy truck with a 72 350 engine which has crud and rust in the fuel tank which clogs the filter. I am getting ready to order a replacement tank. I am probably going to order from LMC as I have ordered
parts from them before. I changed the fuel cap several years ago to a locking cap and notice corrosion and rust on the cap when I remove it. I just noticed on the LMC site they list a non-ECC cap and a ECC fuel cap. Did I screw up and cause the problem by getting the wrong cap or is it just the old tank is typically at the end of its life cycle. I don't know which type cap I put on the truck. Any opinions?
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The cap is not causing the rust. You just want an old vented cap rather than a newer unvented one that could develop a vacuum in the tank.
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