I agree with several of those posts...
Definitely set the tank perpendicular to the floor joists. In fact, the more joists you span with that tank, the more uniformaly distributed the load will be, so tell your wife/girlfriend/parent you need a MF Monster tank! Over a load-bearing wall would probably be best, though.
House age would be an important piece of info if the house has wood joists. Rotted, termite chewed joists will not hold a big tank for long... If that part of the house is only 6 years old, try to find where it attaches to the older part of the house. Make sure there is solid joints in that area, especially if the tank will be near it. Big tank weight over a bad joint/joist could easily put cracks in the wall paint/drywall or even worse.
Good luck! Let us know what size you go with!