The original leaf spring pads will need to be removed from the axle tubes. There’s a multitude of removal methods, but keep in mind the less heat used the less chance of heat warping the axle tubes. For what it’s worth, I use a cutoff wheel on a die-grinder. It’s kind of a pain but a lot cooler than torches or a plasma cutter. A standard grinding wheel can be used to clean up the rough stubble left on the tube—just go slowly and don’t get the area too hot.