I know there is snapage in the future...and it wont even take much.
I bought the same exact kit from my local Advance auto I drilled an extra hole thru the center to brace it. And even with that there was too much space and in some light wheelin my LR snapped leavin me to have to un hook the right side and chaining my rear springs to the frame rails and limping home.
My permenent fix...an inexpensive add a leaf kit from Rough Country.
I also bought a set of stock replacement spring packs from JC Whitney for 120 bucks a side. To go with that I bought a set of 1 1/2 lift gresable shackles from daystar. Those were installed with the add a leaf kit at a later date. I.E. I put the add a leaf kit in the stock spring set till i got the new parts a few weeks later. Then added the add a leaf kit the the new stuff upon installation.
http://www.jcwhitney.com/ALLOY-STEEL...3409_10101.jcw
http://www.jcwhitney.com/ROUGH-COUNT...3409_10101.jcw
http://www.rockymountainsusp.com/W_shackles_Toyota.HTM
all together I got about 4 inches of lift and by the time everything settled a little over 3. And its a far more reliable set up than those shackles you installed.