Just for background, I road race my '64 Dodge Dart in the 24 Hours of LeMons. I've just finished rigging up a new transmission kickdown for my 225/A904 with a 2-barrel Holley 2300 (350 CFM). It uses most of the original hardware that attached to the single-barrel rod linkage in the early A-bodies. It was also really easy to install, so I figured I'd share the process.
If you unscrew the adjuster clip that attaches to the throttle shaft (CLIP 14-13-3 on this diagram), you are left with a threaded end on the rod that pushes down to actuate the kickdown lever (ROD 14-11-9... the upper one, apparently there are two different rods with that number). These threads are 1/4"-28.
Now you just need a 1/4"-28 turnbuckle; these are a little difficult to find locally, but you can get them from McMaster. I used part number 3082T12, which is a solid-body stainless turnbuckle with eyes on both ends. I think a solid jaw end would be better for attaching to the carburetor bracket, but the only available jaw ends I could find were not solid; they're only made to hold tension, not the compressive load that will be applied here.
I also picked up a small plastic bushing (PN 6627K13) to connect the eye bolt to a 1/4" hole on the carburetor throttle bracket, but the OD of the bushing ended up being too small for a tight fit in the eye. There are other bushings on that page with larger ODs; they might be too long, but you could pretty easily trim to fit. In my case, I just used a tiny piece of scrap rubber gasket material between the bushing and eye, then I attached it to a convenient hole on the throttle bracket. This is obviously the left-hand threaded eye. The right-hand threaded end is not used.
Or at least it wouldn't have been used if this turnbuckle body was long enough! As it turns out, I didn't measure correctly and this whole assembly was 3-4 inches too short. At first I thought I'd replace the turnbuckle body with a female-threaded connecting rod, but I wasn't sure about the right length. If those things were hollow all the way through, I probably could have used the 12" long option with no hassle, but they're not. I also couldn't find a 3" longer left-hand-thread eyebolt in 1/4"-28; who would've guessed? What I ended up doing was cutting the eye off the unused right-hand-thread turnbuckle end to get a 3" long piece of 1/4"-28 threaded rod, then I used a coupling nut to connect this short piece to the end of the existing kickdown rod.
[Removed broken image link; updated pics are in the post below]
If it's not clear from the picture, the carburetor throttle bracket to which all this crap is attached rotates clockwise when the throttle is applied. This pushes the rod assembly downward. The kickdown assembly could still be about an inch longer, but I just extended the sliding adjustment under the car and still had maybe 1/2" of slack left that I could have used. You can then use the turnbuckle for fine tuning. I haven't finished dialing it in yet, but I suspect the total throw on the mechanism should be a bit longer as well. That is, if I adjust it so the lever on the transmission is pushed fully backward at full throttle, it may not come forward as much as it should at idle. That could be fixed by fabricating a small bracket that bolts onto the carb throttle bracket, effectively increasing the radius at which the kickdown linkage attaches. As it is, the hole to which I connected the upper end is about 1" away from the center of rotation. Perhaps your carb's throttle bracket has room to drill a hole slightly farther away from the throttle shaft. I don't particularly care in this case, because racecar.
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