You won't be able to build a full tub from panels, but you can replace most of the exterior skin with replacement panels - some cut down from PU bed panels. Other areas aren't available as replacement panels, like the A Pillar, windshield frame, B Pillar. The cross braces and body mounts aren't available either. You're best to start with a decent tub.
Here's a project that's available on craigslist.
http://seattle.craigslist.org/tac/cto/3279569310.html It's a long way from you, but something similar might be a good starting point. Also keep in mind, shipping is cheaper than repairing rust.