Most guys don't move the springs in because the tires are too close to the spring. They move them in to get wider tires on the car. When you are putting an 8.8 in any Mopar, you need to weld the spring mounts on regardless, might as well put in the work to do it all correct and get the look that you are after.
They shorten the axle to get the best geometry out of the setup as, ideally, you want the mounting point to be in the middle of the wheel well. Also, you can not get a wider wheel in a Dart wheel well if you only have minimal space. You need to shorten the axle to give the space needed to clear the quarter panel when mounting just about anything wider than stock wheels on a Dart.
Dart and Duster are not even comparable in any way when it comes to the fitment and measurements of a rear wheel. You can not use the measurements from a Duster to mount wheels on a Dart, and vice versa.
