stiffening pieces to weld into frame

The main advantage is to allow more positive caster. The original specs call for almost no positive caster, and most modern alignments call for +3 or more, which you typically can't get without offset bushings. The additional caster makes the car more stable at speed. I have close to +5 degrees on my Challenger, but I have Hotchkis UCA's. And you wouldn't want to go that high with manual steering either, as it will make it hard to turn.

This is how they should be oriented for more caster, its pretty much opposite of how the instructions tell you to install them (they were intended for something else).