Dunno what the unloaded arch is, and we're having a snowstorm here at the moment so excuse me if I don't shovel a path to the shed to measure for you.
Having done this, here's what I found. Don't use the "long" (second) leaf. Take apart your donor pack, cut the bushing eyes off the main leaves just short of where they start to turn up to form the loops, and use those. Put them directly under your main leaves, which you can leave in the vehicle, then put your second leaf under those (moving the second leaf down to the third leaf position).
Using old donor springs in an old Cherokee, I gained a gross lift of 1-1/4" doing this. Other folks with newer Cherokees and/or newer donor springs seem to gain around 1-1/2". The one I did was sagged about 1/2" so I ended up sitting at 3/4" higher than stock, which was perfect. The spring rate was increased for better load carrying, the ground clearance was slightly better, and it still rode very well.
If you are really sitting at -1-1/2" now, putting in just a second main leaf will just about bring you back to stock height. If that's all you want, then that's all you need to do. If you're looking for a bit of extra height, then I'd suggest you install the main leaves from the donor pack plus the second leaves. Mark the tips of the second leaves before you disassemble the donor packs to show where the third leaves come to (actually, the rust pattern will probably show that pretty clearly anyway). Then cut the tips of the donor second leaves so the ends will fall halfway between the second and third leaves in your existing springs.
Then when you reassemble, the order will be
-- Original main --
-- Donor main --
-- Original Second --
-- Donor Second --
-- Original Third --
-- Original Fourth --
This will add about 1/2" to the thickness of the spring pack. The OEM U-bolts are long enough to handle one additional leaf, but I don't think they're long enough to handle two, so you'll probably need new U-bolts. Get good ones. The factory ones are 12mm, which is almost 1/2" but not quite. The ones I've seen in the chain stores are 7/16, which is smaller than 12mm. Go to a spring shop and get 1/2" u-bolts made up. If there isn't one near you, get in touch with Husky Spring
www.huskyspring.com