i think that it will be fine.
400 X 8lbs per gal = 3200 then / 6 = 533lbs. and your rock will weigh less per sq ft then water so it will be less then 533lbs per foot.
but, you can go to lowes, and get some aluminum or steel that is 3/8ths thick and make a 6x6 square or larger, and this will distribute the weight from one sq in to the 6x6 sq in. which will only be 14lbs per in. (in theory)
i wouldnt place a piece of wood/plywood the size of your stand on the floor, because if there is any splash or incident, it will hold water between the oak and plywood.
another option is to get 2 - 7ft strips of oak, and have them polyed on both sides and place your stand on it. that way the stand will dent the strip and not the actual floor under it. (what a few dollars verses replacing the floor later)