Is your car an original 4bbl or 2bbl carb engine?
The intake is much higher than the stock one, meaning the cable needs to be longer or the bracket needs to be higher to compensate. Second, you may have a 2bbl cable on top of that. Just as important as the throttle cable being long enough at idle, but you must make sure you are getting 100% throttle opening at the carb when your foot is on the floor as well.
I have a Lokar throttle and kickdown linkage setup, works flawless, takes a little time to setup, but is worth it when you start messing with different carbs and manifolds. It bolts to the driver's/rear most carb stud. The throttle cable is cut to length and has two separate adjustments. Throttle return spring is on the same mount and bolts to the lower part of the carb linkage.
Kinda hard to see, but this is how my engine used to look. I have a 650cfm AVS (same base carb) and Performer RPM intake (same height as the airgap).
(plus I noticed the vacuum port was plugged on your manifold, where are you getting the vacuum for your power brakes)