I don't have a clue as to why they are saying to cut off 2" off the leaves.
But guys, this is not rocket science. This is ancient technology that was common practice in the 60-70s. Old drag racers will remember it.
Making "Mopar Super Stock Springs" out of stock leaf springs on other cars has been done for 45 years. It is such an old idea that it has been forgotten by most and disavowed by the companies that $$$$ sell slapper bars and other suspensions.
I run them on my car right now, and have since I got it 17 years ago.
Oh, and don't forget urethane bushings everywhere, and do eliminate the rubber around the leaf spring.