Early Mustangs didn't have a lever arm running from the carb to the kick-down lever on the transmission. Rather, they had a arm extending down on the gas pedal bellcrank which was connected via a cable to the transmission. I have found these kick-down lever arms mostly as a welded addition to the pedal assy, but also as bolt-on. It is possible to use them to activate the kick-down, but I believe it also would require a different kick-down lever on the transmission, internally and externally (can't recall the details). Is it possible your drivetrain (engine & transmission) have been changed out to a later model?
Tom Kubler, Long-time Mustang Enthusiast & San Antonio Mustang Club Founder