shim the stand first. make sure the stand is level and then put the tank on top and make sure the top of the tank is level. if not, use the styrofoam under the tank.
isn't it going to be hard to shim the stand you have? doesn't it have those little ball type legs? i can't remember lol. if it does and you cannot get it level, i would buy one of those thick rubber mats from home depot, and put it under the tank, that will help support it.