kickdown cable making problem

Doing that was making the cable housing flex and making the adjustment too tight.
My setup is 100% improvised. I use a Lokar throttle cable along with my original throttle rod and bell crank on my van. My pedal has a rod that passes through the floor to a bell crank, then a long rod that runs back alongside the frame rail (with guides)
The Lokar cable is secured to a mount, the inner wire is connected to the rod end and the cable loops up on the backside of the engine up to a bracket on the intake that it mounts to. The inner wire then connects to the carb.

Originally the cable housing would flex when opening the throttle, be all squiggly (for lack of a better term) and I couldn’t get to full throttle, and it was too stiff.

What I did was use a length of brake line and bent it to the radius I needed from where the cable connected to the throttle rod below at the frame with a section of hose (to accommodate engine movement) up to the cable mount on the engine. Then I ran the cable housing through the brake line and hooked everything up.

Short story long... for your kickdown you might try using some brake line and bend it in a manner so as to replicate how the cable situates when hooked up. Feed the cable into the brake line, then hook it all up. How much brake line you would need depends. Might just need it for the big radius where it loops around at the back of the trans, maybe longer. Perhaps the whole length. Would be easy to fabricate regardless. That WILL prevent any flexing. 100% guaranteed

I have a full manual valve body so I don’t use a kick down (I did on my van originally but I suffered the same issue as you mention for some reason) but if I did again I would look into this:

BPE TorqueFlite Kickdown Cable Kit - Bouchillon Performance Engineering