The best thing to do is to buy a reproduction 1967 Ford Mustang shop manual. Also, check out the Ford Six forums. There is tons of information on the Ford inline six cylinder engines from the early 140ci I6 to the 300ci I6 and more. My 1965 Mustang has a 1968 200ci I6 in it with a 1968 C4 transmission. My kickdown cable was never attached but the transmission has always operated fine. I am not suggesting that you do not need to worry about it because it should be fixed. I am sorry that I could not be more helpful.
I'm not too familiar with the I6's, having never owned one, but to the best of my knowledge it's like this:
'65 - all used a rod.
'66-'68 - V8's used a cable, I-6 used a rod
'69 - '73 - all used a rod.
The "downshift control cable" is a nice-to-have part but you can certainly drive without it. That being said, you can find a repop at National Parts Depot, DT Mustang, etc. for a pretty reasonable price (approx. US$15)