My floor was out that much too, I'm a contractor/carpenter, that much out is pretty bad.
Mine was also on uneven stone, so I had to lay a crete pad, but most likely you can either just shim it, or take 2x4 and rip it from say a 1/4" to 3/4" in 4' lengths so you can take out that half inch variation.
If your tank is trimmed you mainly need support at 4 corners, if not trimmed it's much more critical, but either way I'd make sure it is supported at center so your stand doesn't sag.
So I'd rip a 2x or shim every 6" or so