Your car looks like it's sitting a little high in the back. I'm assuming that's the look you're going for.
I would research the installation of the shims you're looking for. Thru bolting the shim in place would seem safer, but if the instructions say no, then follow them. The Jeep Wrangler YJ series has spring under design just like RWD Mopars. The shims function exactly the same. The shims I used slipped on top of the locator bolt to hold them in place while you bolted the spring to the axle. The locator bolt kept them from getting "spit out". If you changed your locator bolt to some other design due to the add a leaf then you may need to investigate another option. The U-bolts keep the shim from spinning around the locator bolt and coming out the side.
So to understand your setup, what kind of springs did you use before the add a leafs? I put XHD springs in the rear and still had to put air shocks to get my car to stop sagging and sit level. I have a pair of used SS springs that are going in this year just to see if I can get rid of the air shocks.
I had stock springs in the car, which were soft and, in my eyes, saggy. With anyone besides me in the car it bottomed out on bumps etc....stuff banging around under it....and that was just two small kids in the back.
Looking into spring choices, seemed like the ready-made options would not be much better, or just give me a stock (low) look......I wanted the car to have a little rake.....I had read that super stock springs actually make the car sit crooked in back as one is stiffer than the other to counteract launches. I read here as well that you could have custom springs bent but that sometimes it is a crapshoot.
So, not sure what springs to get at the time, I lived with it for the first summer I had the car. Then that winter I decided to follow the lead of another member here and try the add-a-leafs temporarily. For $80 or so it was a cheap gamble. I removed the half-leaves and replaced them with the add-a-leafs. Replaced the pack bolt with the new one at that time as well as new u bolts.
(it was just last winter that I did that and I just cannot remember how the pack bolts works in relation to the plate etc....hence my questions.)
Lets just say the add-a-leafs took care of any sag or bottoming out. LOL....I also added 1" torsion bars up front. The thing handles like its on rails as long as I dont push the speed TOO hard in corners, cause then the rake comes into play. But its not a rally car anyways...its a cruiser/funmobile.
It rides firm, but not like a dump truck. I actually like it. That, and I have lower profile tires on it than I would like....going to a bit taller ones soon I hope. Also have KYB shocks all around (had to go with dodge pickup shocks in the back for the extra length).
I compared my car to a guy's 72 'Cuda down the street...he has super stock springs on his. My bumper sits a couple inches higher than his.
I think i might eventually go with springs that drop her down just a tad IF I can figure out what to get.... but for now it works for me...I like some rake.....and I have gotten great feedback at car cruises etc....LOL ...I want to wait til I get new tires and rims so I can see where I am at......the shims are just to correct the issues right now and hold me over.