Hmmm, I wrote this: "...a column of water only 8 feet high, 1 foot wide and 1 foot deep weighs about 500 pounds." I believe this to be correct, since 1 cubic foot of water weighs about 62.5 pounds (just imagine stacking 8 of those on top of each other). I used this 8 ft by 1 ft by 1 ft column in my subsequent calculation.
My apology. I mixed and matched your numbers. You are correct.