I would level the stand, so put the shims between the floor and the stand. The combined weight of the tank and its contents will be transfered through the stand to the points that touch the floor. The highest pressure will be on the wooden stand. If you shim under the aquarium the weight distribution will be on the glass and sure to cause breakage.
Have you seen this thread? There may be something you can use in here.
Sub floors & preparing for your new aquarium