1/2" is a big gap under the tank on top of the stand.... start with leveling the stand as best you can in between the stand and floor and that 1/2" should decrease. Even on my worst un level tanks I maybe have 1/8" or less to fill in with shims in between the tank and stand top. If floor shims do not solve the 1/2" gap you have, id rebuild/rework the stand top itself. The piece of 3/4"-1" wood under the tank will support more than you think, even if you shim under that piece of wood its spreading out the pressure across the whole sheet not just the point/area of the shim. If your stand is built well it can handle floor shims up to 1/2" or more no problem to compensate for un level floors. I actually raise all my stands up off the floor anyway using the thin 4" solid cinder blocks or 2x4's on 4 corners and other leg supports. My basement floods a lot from hard rain and floors are pretty much wet 24/7 in a fish room so raising them up a bit lets the floors dry without stands rusting or rotting. suspending the stand a bit makes leveling them very easy, same principal applies right off the floor though. Its ok if some spots of your stand have air gaps to make it level. better the stand than the tank on top.
