I think that you are going to want to place the slider box in a position where your springs do not run out of sliding space at either end of the slide whether it be at full spring compression or at maximum expected droop. No one knows how much arch your springs have or what suspension goals you have specifically--maybe your springs are so flat that you won't need much room for the slider on its compression stroke, and you are going for good amount of droop on a teeth shattering, bottomed-out Toy lowrider (and the box sliders enhance this all the more), or maybe you have heavily arched springs, and you want a lot of up travel, or maybe you want a very balanced leaf-sprung suspension that works really well, and you want to minimize ride height and COG with the slider boxes. I'be curious to know what you wanted your cruiser to do in the end.
I think that using high quality custom springs from Deaver, as Wilson did, will work best for this concept, and if you're gonna do this, go full tilt and get right the fiirst time. Also, you could order up a reverse-eye set of springs, which keeps you still a smidge lower without reducing spring arch-- you just gotta nake sure that the upper ceiling of the slider boxes you will fab/use do not interfere with such a spring eye.
From what I have read elsewhere, anyone doing this would be hard-pressed to use anywhere close to the entire slide length on the slider boxes, such as those made by Timmay at LLI, even while getting great overall wheel travel lengths. That being said, I think that you probably should install a set of bump stops, and depending on what you can get for droop, maybe even some limit straps if necessary.
The nice thing about running the sliders up front (below the radiator) is that you will not be stressing out your Driveshaft/U-Joint/pinion angle on full drop like you would do with shackles mounted rearward under the cab. You can also run a traction bar possibly that generally mirrors the arc and mounting locations of your leaf springs.
I would definitely think through carefully your set up and spring choice beforehand. Good luck!