I'm in the process of installing this kit right now in my 68 Barracuda and I also had the issue where the temp control slider only had half the range of travel and wound up having to create a new loop in the wire.
Does the temperature slider work the water valve as well as a blend door inside the box or does it just control the water valve? Reason I ask is that I noticed that it only takes about 25% travel with the temp control slider in order to have the valve be fully open or fully closed. It's as if the potentiometer has too wide a range of resistance. Either that or the rest of the range of the slider controls a blend door inside the box but I never heard anything moving inside the box when I continued to move the slider the rest of the range.