The C-hub is what you are modifying. The original hole from the kinpin is a hair too thick to fully tighten the LW kingpin. By relieving a touch of mat'l you will have just enough to tighten but not weaken.
I have avoided the narrowing of the tread as the less width you have the less traction. Also by putiing all of the spacers behind the wheel you widen the track but also ruin the scrub radius of the kingpin.