I don't know if a 440 6 Pack that was not made for racing and made more as a "toy" for Mopar is that far out reach. After all they made the Diamante show car that was pretty radical.
I wouldn't put it past a guy back then in the Mopar office to think, "Hey, if a 340 6 Pack T/A Challenger is cool, what about a 440 6 Pack that we already have lying around?" We know what made it to production but we'll never know all the weird creations that were in the back room and maybe one or two escaped into the public. Maybe this is just a fantasy of mine but I think back then anything is possible would be cool to think that.
About 4 -5 years ago I went to get some magazines off an older gentleman and got to talking to him. He said his son worked for Ford and was test driving a Mustang with 1,000hp in developmental stages. Never know what is cooking behind closed doors.
Not saying this car in particular is a real factory made car but just saying like others have said never say never. I'm no expert but would be cool if this really is a factory made car.