They sure look like they were designed to rust out, don't they? All those nooks and crannys to hold mud and road salt are just asking for trouble. The additional pieces you'll likely need are rocker panels, cab floor supports, inner cowl panels, inner-to-outer cowl panels, and outer cowl panels. All the usual suspects carry them.
See mine:
http://www.chuckthetruck.homestead.com/page1.html