OME Springs and Rear SLS
 

So if I get OME HD (779's) Springs in the front and replace all the shocks with OME's, do I need spacers in the back for the SLS?

RE: OME Springs and Rear SLS
 

If you want to get the height balanced out you need to buy an RTE spacer, http://www.rovertym.com/
You could also raise the back by messing with the height switchs but that will just make your truck ride way to firm and blow out you bags early.