If your current leaf springs are shot, a great temporary fix for a smooth ride (while loaded down with gear) is the Zuk mod. Look it up on Yotatech...there are a few different versions, but the simplest form is taking front coil springs from junkyard '97-01 Jeep TJ and wedging it between the rear bump stops. It will level out the rear and give your bump stops a little breathing room, and it's a quick mod to try while waiting to do the 63" Chevy springs.
The battery location is pretty good (you can even stuff dual Odyssey PC1200's in there), but if you want to go further back and lower you could try what Ramblewriter did. They cut a hole under the 4Runner (I believe above the driveshaft or rear end) and used a baking tray hung as a battery tray. Sorry, I can no longer find pics of it or their description so this is kinda useless.