Might've been a J-code car, but it certainly wasn't packing it's Hemi anymore. John Schneider has been dispelling rumors on his Youtubes and though he's maybe an armchair mopar guy at best; he can verify that for 1st Unit filming, meaning filming with the Primary cast, not a single car was a Hemi car. The "Hero" car was generally (haha, Pun!) a Big Block car, but could be 383 or 440.

More often than not, the stunt cars were 318 cars. They required less ballast in the trunk to keep the car flying level through a jump. Less weight in the trunk kept the car from "folding" upon landing as well. That fact there leads me to believe there were no Hemi cars used. That big Hemi would make a Charger nose over big time and the amount of weight in the trunk necessary to level it out would be excessive probably negating any performance the Hemi may have had.