This entire subject is quite difficult to explain in words, without visualization assistance.
Here is another post I made on the subject, which has animation videos in them of good and bad bumpsteer behavior.
Look at the bumpsteer curve below:
If we had a car with "perfect" bumpsteer behavior, ie none, the graph would be a vertical straight line lying on top of the Y axis. This means that no matter how far the front tire went up or down (+3" to -4") the toe would always remain at 0 (no change). Now if you take this car and lower it, what have you changed about the bumpsteer geometry? Absolutely nothing! All you have done is moved the zero point in the suspension travel curve maybe 1.5" upwards along the graph, which is a perfectly straight vertical line.
In reality, we can't build a suspension with perfect bumpsteer behavior. If you did, it would compromise other aspects of the suspensions performance too much. With any suspension, the bumpsteer curve is going to have an area of the curve where it is flattest. You want to adjust the location of the various joints in the steering system to put this flat area of the curve in the range from ride height to about 2" of bump travel. See this link for a good example of this:
Each of the graphs has three curves. I assume that the person who made them adjusted the height of one of the steering system pivots to three different locations to get the three different graphs.
Once you lower a Mustang, you have shifted the flat portion of that curve to a slightly different area of the graph. It only takes a small adjustment of the height of the outer tie rod to get the curve back to where it was. If you were to lower the car and then reset the caster back to the setting it had at stock ride height, the bumpsteer curve would then be virtually identical to the original!
The point is, lowering the car does NOT create some massive change in bumpsteer behavior on a Mustang. So if all you are doing is lowering the car, there is absolutely no reason to change the spindles to fix a bumpsteer problem. It is guaranteed to make it much, much worse. The only time you will need to change spindles or install offset steering rack bushings is when you have made some large relative change to the vertical location of any of the following four pivots: Inner tie rod pivot, outer tie rod pivot, balljoint, FCA inner pivot.
Sometimes changes to the location of these pivots are obvious. When you buy a k-member that advertises that it raises the FCA pivots 2", you know about it. Sometimes they are not. When you add caster to the front suspension by moving the strut top back 1/2", you are raising the outer tie rod pivot as a result.