People build '69 Chargers from an A pillar, cowl, and a rocker panel because they have the VIN. A car like I described sold for $13k! People take cars down to similar states when they replace sheet metal, so maybe it's all in a gray area. Think about it like this, you buy that Lil Red in '79, and in '80, it's involved in an accident wrecking both fenders and the hood, bumper and grille. It goes through insurance and all gets repaired. Is it original? It's still a Lil Red, but it doesn't have all it's original sheet metal.
As long as it's got most of it's body panels still with it, I don't see anything wrong with getting a better cab, if yours is that far gone. Floors get replaced without concern, so do doors. Where the line is, is up to you. I'm all for keeping a Lil Red on the road!