Radiator shops will sometimes do fuel tanks for restorations. I'm more a DIY type of person so this is what I would do. Remove, empty, inspect the seam for wetness/oil, then power wash both the inside and out. Carefully examine both the inside and outside especially around fittings and low area inside where water can collect. Sometimes this requires a special inspection camera depending on the tank. If the tank is clean you may not need to do anything further. If rusty or corroded, then a chemical cleaner is warranted, followed by a sealant such as red kote. Restoration specialty retailers such as Eastwood, Jegs, and Summit all carry kits with the necessary items and information related to making a steel tank like new. Fuel lines are generally copper, are rarely need replacement unless physically damaged. They may develop a green patina, but that is not harmful. Rubber lines should be replaced if there is any question about there condition. Add a good water sep filter, and the system should last for many years to come. Good luck.
PS. Poly tanks are great replacements, but it will likely require some fabrication to make one fit in the original space. A new steel tank is possible, but if I had to customize a mount for one, I'd opt for the poly tank.
