get yourself a Ni-terne steel tank and if you want an extra measure of protection, put some clear coat over it. The tin/lead coating over the top of the nickel takes paint really well. Just scuff it up and wash it before you paint. The slippery feeling of the tank is due to the lead part of the coating. The Ni-terne steel will probably outlast you, but will develop a little surface rust at the weld seam where the two halves are welded together.
I have new tanks on my '77 K20 for 6 years now and they still look brand new. I did not clear coat over them. I have driven it plenty in winter and taken it on 3 seasons of hunting.