I would think a 340 4-speed wouldn't be as limited as a 40 model run. '70's gas crunch so smaller engines were more popular and I'd think the 4-speed was there to keep the muscle car feel. I have s '74 Charger Rallye that was factory with a 318-2bbl. And a pistol grip 4-speed! Guessing it's pretty uncommon in itself.
RT