I am looking seriously at doing this also. Many CNG stations here and the price is around $1.43 GGE right now.
To the OP: You don't need to do anything with the diesel fueling on your truck. The amount of fuel will be controlled by the throttle pedal. When CNG is flowing you will only need to push the pedal half as far to get the same power. As Agnem said above there is no throttle plate in a diesel- its full air all the time.
My system will incorporate 2 extra safety features. There will be a shut off valve tied to oil pressure and boost so that no CNG can flow at idle or when the truck is not running. I consider this to be the most important part. I will also have a CNG tech inspect my installation prior to putting any fuel in it. These guys are relatively easy to find here in Utah.
Another thing you need to consider is that CNG can easily detonate at high flows and loads- this will generally mean a blown head gasket. My CNG system will be set up to deliver the highest CNG/diesel ratio at highway cruise speeds. As you demand more power you actually need to taper the CNG off or your peak cylinder pressure will get too high. Its readily accepted that 50/50 CNG/diesel at cruise is safe, but at any other load you should not go over 30/70.
CNG has a compression detonation factor near 15:1, so you have to take care running it in one of these old 21:1 and higher trucks. It will effectively advance your timing too far.
Propane is worse with a 13:1 detonation factor, which is why you hear so many horror stories from guys that have tried it.
Good luck and keep us posted.