lower control arm shim

I'm going to be installing a 4.5 lift but I'm piecing it together. My question is about shimming the lower control arms out to get the correct length. I have a set of Pro Comp lowers but they are only 16'' center to center and I'm being told they must be 16.5 long. I see that in my Cherokee now there are a few shims but there is not a lot of room in the stock hole now and I'm wondering if they came with shims in the stock suspension. Hopefully someone can give me the info I'm looking for. Thank you for your time.