1965 Dart GT: help on diagnosis?

So I have a 1965 Dodge Dart GT with a 273 Super Commando (no, the engine isn't original to the car). First of all, I am 17 years old. My dad and I got this car for free and have been working on it on and off for the last year due to money restrictions. One of the many problems I have come across is the car won't kick down; and this is a big issue because when finished, we plan to race it. My dad has been around cars his whole life and says it's a shifting cable, but I can't seem to find one. The Dart has a 3 speed automatic, and I don't really want to part with the shifter because it is original to the car, but I just wanted to see what all the experienced Mopar Guys have to say! Thank you all!
Your kickdown linkage should look like this....1104.jpg

Like what was said...DON'T drive the car without it...you'll burn the trans up!!!