A RSK with S/D springs does end up riding VERY nice, but, it also ends up TALL, and you have to futz around with axle location, as the S/D springs are longer on the front by about 2" with no load, so, it ends up pushing the axle back 3-4" once the weight is on them. You need to push the spring bracket as far forward as you can, and I'm going to end up using a zero rate block on mine to finalize the axle location.
You must be registered for see images attach
You can see where the axle ended up on my truck, and things are sitting about 2" forward of where you'd have put the bracket with stock springs.
It also ended up being a good 6-7" taller than a stock F350, and those are stock S/D springs. The truck has a 3" body lift also... Those are 38.5's on there, and as you can see, I'm not exactly hurting for room.