Funny, a buddy of mine has YJs under his with Rancho shocks. His rides mushy, but it won't kill your butt to ride in. I've got full size Jeep Grand Wagoneer springs under my lwb sammy, rides, flexes, and carries a load like you wouldn't beleive! Problem with using these springs on a swb sammy is, you have to stretch the frame a lot! My rear shackle hangers are at the very end of my frame, which is 9" longer than the swb sammy from the axle back. My fronts are on a 6" frame extension. The front springs are 10 1/2" longer than the stock fronts and 1 1/2" longer than YJs. My rears are 17" longer than the stock sammy rears, 11" longer than YJs!
On a stock swb sammy, I'd recommend running with YJs all around, you'll be very impressed with everything!