They can be used for either, but be careful not to lift the locator tangs out of the slots in the chassis. I have found that shimming above the riser block, but below the cross plate is better. You can also put washers under the cross plate where the 4 screws fasten it down,If you go this route, be sure to put a shim between the upper suspension arm holder and the wing, or the wing will have up and down play in it.
You are correct in seeing that the shim will not allow the wing to fit if put in this area, I have cut the u shape out so the wing can slide in.
Hope this helps.
Sorry to jump in, but Brian is on his way to Vegas. I will be joining him there tomorrow.