When I converted my 68 Cuda from 2 bbl to 4 bbl, I just made a little extension for the kick down lever like this:
All you have to do is just get some round or hex stock in about 5/8", then drill a hole in the center for a 5/16 - 24 thread all the way through. You will just need 1" - 1 1/2" length for the female portion shown here:
Here is how the assembly fits together showing the hidden lines. Now just thread this on the end of the kick down lever and adjust as the service manual states: