I've got a 76 which the undercoating is mostly intact and at some point in the trucks life most of the underside was coated with red lead or some other primer of similar color. This has kept rust to the body which is very minimal, a few small holes in the passenger floor and the lower rear corners of some wheel wells. The frame is in excellent shape. It's a California truck and that is why, though.

There is a '77-79 stepside for sale here with original looking paint and no visible rust. I never stopped to look at it, just when I drove by. Body looks straight too.