It's indeed, a 383 with 2-fours on top; a solid lifter motor with .445/.445 lift at the valve, and a de-factored NHRA "Stock" HP rating of 320; just 6 more than a '71 Duster 340, which has a single 4bbl, hydraulic lifter, motor.
Somebody (Messr.'s Bennett and Widuch) did their homework!!!
LOL!
I think this may be the motor -type that was in a notorious MoPar station wagon back in the '70s or '80s, called "THE BEHEMOTH." Could be wrong about that...
I also seem to remember that it was never an assembly-line option; just an NHRA-legal "dealer-installed-option" from years and years, ago...
Someone correct me if I'm wrong, but this could have been the original "paper car" engine.