I have not changed my front springs, which are 4" USA, but I have changed my rear 4" USA springs.
The front broke in nicely and dropped about an inch by my 5th run. I remember this because I had to recenter my steering wheel. The only thing I would change with my front is improve my shackle angle.
As for the rear, I designed my suspension around the fact that I never carry loads greater than about 100-150 pounds. The less weight I carry the more happy I am. I even remove my tail gate to conserve weight for short day runs.
So what I did to my rear leafs, is I removed two leafs from each side, so four leafs were removed total. I first removed the lowest leaf, and then I removed the lowest leaf again. Get it?
![Slap! :slap:](https://board.marlincrawler.com/Smileys/marlin/slap.gif)
I lost over 2-inches of ride height, which fit perfectly the 2" pivot blocks.
My front flex is limited by my oil pan right now (drag link), which also prevents the tires from hitting my cab, which is nice. The rear works very well, as I can get the tires to go up into the fenders by almost two inches.
BigMike