Aluminum doesn't get soft, it hardens after time. Corrosion can be an issue but only if it's exposed for a long time. You can get rolls of coated line at any AZ but I don't know if any one else carries it. Stainless is better but harder to work with. Fortunatly one of the guys that hangs out here gets it from work for free so we're stocked up for now.
Classic Tube or Inline tube has them. Best to call, their catalogs are not that clear as to which line is which for these cars.
And they will be expensive. If you want it factory looking, it's worth it, but if you just want it running, then bend you own. The J bend and the bottom is tight.