Different brands of idler arms often position the drag link in different planes of operation in relation to the pitman arm. And the exact position of the idler arm mount on the 'K' can vary from car to car. All of that changes the arc. It is nearly impossible for most of us to know where the ideal position of all of it should be. And how can we be sure that the pitman arm is in it's correct plane. To say that bending an idler arm that may have the wrong angle to begin with is a bad fix, may not be true. I am not souture that bending the idler's mount on the 'K' to correct an improper angle on the idler arm itself is a better solution. And some folks may not be anxious to burn the paint off their 'K' member. But to each his own.
My background in suspension includes over 30 years of suspension/frame work for a living. You are correct the steering box could be in the wrong location. However I haven't ever seen one off a significant amount unless the car had been heavily wrecked. Idler arm brackets however are common. Whether welded crooked to start with or bent. Many years ago I came across this issue . The front end of the car would move in a circle over bumps. When it was in jounce it would go right. In rebound it would go left. Car had 22k and had not been wrecked. As a young mechanic I pursued a fix. Reading about suspension geometry I realised it had toe change (bump steer). Thinking nothing of it I heated and bent the idler arm. Car drove much better in a straight line as the right and left toe curves were similar. One day my wife pulled the car into our driveways. I saw something hanging from under the car. When she stopped the wheels were straight, everything looked fine. I watched as she turned the wheel. Imagine watching the idler arm swing down about 2" off the ground when turning left. And rising up nearly hitting the frame turning right. After talking to an old Chrysler dealer mechanic I installed a new idler arm and straightened the bracket. This fixed it (still own the car). If the idler arm is moving the position of the center link, it's the wrong or incorrectly built arm. The ideal position would have a parrallelagram formed by the inner tie rod pivot, outer tie rod pivot, lower ball joint pivot, and the center of the lower control arm pivot. Obviously the 2 inner tie rod positions are located by the height of the center link attachment points at the end of the idler arm and pitman arm. As a close starting point if the center link is parallel to the chassis thru its travel most cars will drive pretty well. It can definitely be tweeted to make it perfect. But for most applications this is unnecesary. Hope this explanation helps.
Doug
