The best thing to do that I can think of would be to check out where the structural support beams are on your house. Just check out the blueprints. Your safest bet would be to find a load bearing wall on the first floor of the house and see if any of your rooms on the second floor would be setup so that the tank could rest very nearly ontop of that load bearing wall.
With regards to the weight specifically, I'm not sure. My largest tank is a 29g and it is on the second floor. With everything I have sitting in the tank stand, as well as heavy furniture along that wall, it must be well over 500lbs in that area. It is sitting directly ontop of an I-beam though, so I was never worried about it at all.