Just to be sure, take some measurements of the stand to be absolutely sure it's the floor. 2" off from end to end on a 7ish foot tank seems like a large grade on that floor, like it seems that you would notice the slope while walking on it.
I do agree that the stand needs shimmed along the length of it, so you will need different thickness pieces. I would think about every foot would be fine. If you really trust the strength of the stand, then you could just shim it under the vertical supports.