First, wingspan is a crude metric you shouldn't care about, because it takes into account shoulder width and finger length which aren't relevant to arm/height proportions. Your arm length is what you should be focusing on. When your arms are at your side, what is the distance between your wrists and the floor?
I'm concerned about you wanting to do 5" as that is a massive amount of lengthening and could throw off all kinds of proportions. Have you done mockup photos of your body with 5"? Those are really the only way to see if your proportions will be tolerable.