I would second Old Telly with the advice to make sure you have a reliable way to monitor your water temp before blocking your grill.
Here is a picture of the cheapest "push-pull" cable I have found. It is from JEGS racing products and was about $9 US when I bought it last year. It only has a 2" "throw" so you will need to make a lever to allow more than 2 inches of movement. Also being low cost, when the cable moves in and out, it also twists, which can cause binding.
If you what to spend more money, JEGS also has heavy duty push pull cables with a 3" "throw" for around $50 US. Here is an example of the better quality cable being used to operate a manually adjustable grill block.
Here is a picture of how the cable is mounted in the car.
Hope this gives you some ideas fro your project.