31s are more reasonable with that height. The lift will give at least 3.5" but not always more than that. I often hear of people who got more than the advertised lift from their RE, but haven't yet heard of anyone getting less than that - to me it's more important that they don't go under. When I did my 4.5" RE, all measurements indicate that's exactly what i got, so don't expect to get 5" out of a 3.5" lift. For that matter, around 5" is what you want to have to fit 33x10.50s, with about 4" backspace on the rims. Of course, an Add-a-Leaf kist is only going to prop up your existing sagged rear springs so you may not get the full lift because of that...
$600 installed for the AAL RE6020? That retails for $350 so you're giving someone $250 to do what you and a couple buddies could do in an afternoon for the price of a couple pizzas. Better yet, skip the AALs and go to the RE6025, it's the same lift but with replacement spring packs, for almost the same price as you would pay to have someone else do your wrenching. You don't say where you are, but there are probably a few members nearby...