My XR 400 springs were 530mm about. I do not know if XR 600 springs will fit in KDX. I would think that if you got the ones you have in hand into the KDX forks with that much preload your suspension would be hard as a rock and good only if you weighed about 400 lbs.
Net, you got the wrong springs by some error. Return them and start over. The switch to XR 400 springs is a nice move, and you will be glad you got them, when you get them installed. Ride safe John