According to several old time drag racers in SO CA back in the early 1960s that did R&D for Ma Mopar then only factory sponsored racers got the parts to make their original single four barrel cars have that "special" option work
None of those 343 HP optional motors made for Dodge or Plymouth in 1963 either due to the 415 HP and 425 HP Max Wedge and SS option cars being available through the dealers.

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)