There is usually some fine-tuning of linkage when doing a carb swap. The new carbs are sort of "universal" and may not be exactly the way you are used to.
I am putting together a small block right now with that same carb, and the relation of the throttle to the kickdown is a bit different than GM carbs had. It would seem that the linkage attachment points on the new carb are a bit closer together than on the original carb...(more compact, if you will)...and it messes with the cable travel on both the throttle and the kickdown.
You can start by loosening the bolt on the back of the carb that also holds down the throttle cable bracket, and doing your best to rotate that bracket as far rearward as it will go to eliminate some of the cable slack. I had to modify my bracket a bit to get a good fit, so maybe a new bracket would be better to experiment with...rather than messing with the bracket you have?
I also made myself a bushing that goes in the large hole where you presently have your return spring, so I can mount my cable connection there. A washer or 2 of the correct outer diameter might fill that hole like a thin bushing. With a larger washer on both sides to sandwich the bushing in place, you can mount your throttle connection pin through it all to hold the whole mess together.. So the idea is to move the cable connection point forward, and the cable housing mount rearward.
Once you have that set up to your liking, the trans kickdown can be hooked up and adjusted. My setup was good, but it didn't pull the kickdown cable far enough. The plastic clip used for cable adjustment has plenty of travel to make the cable longer, but limited travel for making it shorter. I had to enlarge the holes in the bracket that bolts to the rear of the intake so I could move that bracket a bit further rearward to make my cable a bit shorter.
Adjustment of the kickdown is simple...Lift the plastic clip on your cable adjustment. Open the choke on your carb with your finger, and with the other hand open the throttle fully. While holding the throttle fully open, push the cable plastic clip back down to lock it in place. (it will snap when it seats fully) The cable adjustment should be correct then. There should be a tiny gap between the pin on the carb and the end of the long slot on the cable end....you want to achieve full throttle just before the cable reaches full extension, but you also want the cable to be pulled as far as possible without hindering the carb movement.