check the radiator support and such, especially by the battery. check the gas tank(s) for any wetness. the weatherstripping around the door should be pressed on a body lip that tends to rot away. check the cab floor. the bushings on the door hinges usually go bad and cause the doors to sag. the corner of the cab where the door closes against likes to rot away. the bumper supports and ends of the frame rails can rot pretty good. never really noticed too much else on the frame though and my truck was used for hauling boats out of salt water, so it got it bad! i believe the fenders are metal too.
http://www.lmctruck.com/icatalog/fd/t.aspx