Being that it's just a clip/cab/body sitting on a non-rolling frame, it doesn't matter the slightest bit how original the truck once was. "Was" being the key word. It's nothing like a 427 Corvette or an LS6 Chevelle where the body shell is worth oodles all by itself. When a long bed 305 C10 is complete and super clean, sure it can be worth a decent amount, but when most of the parts and originality are gone, it's really just like any other project truck.
As to the realistic value, it just depends on the buyer and the location. Personally, I feel it's worth much more than 1k but much less than 6k. And that's purely from the presented photos and information. There's no shame in valuing your truck higher than most people would (I know that's how I am with mine) but if the goal is to sell it, that all goes out the window.