Who says Dodge was the first with an extended cab pickup? Yes, It’s only a two-door, and it would have been fairly easy to make it at least a three-door like the Suburbans of that time, but you gotta’ start somewhere.
This obviously started out life as an 8′ bed C10. Ideally, they’d have started with an 8.5′ bed Longhorn, but that didn’t come along until the spring of 1968, and they are–and always were–pretty rare.
The bigger question in my mind is whether this was converted to this extended cab when new, by one of the many outfits that built special pickup/truck bodies, or a later homebrew job. It looks well done, but that doesn’t answer the question definitively.
Nice job, either way. And no worries about the kids opening a rear door while underway.