Never woulda guessed Tom, that's really cool. I'm actually looking to make my first venture into the big aquarium (75ish gal.) world. Just beginning to research all that will be required for that and what I actually want it to be.
Floor. Make sure your floor can handled the weight you're talking about 350-400 lbs in a small area.
My #1 tank is a 75g planted aquarium. As it's in a prime viewing area for guests to our home, I bought a tank that is really a piece of furniture, very clean appearance - no visible wires, lights, pumps, nothing but wood, water, plants and fish. It does keep me motivated to keep things well maintained.
Blue's point is on target. I would consider our house to be shoddily built, the minimum floor strength to meet then code (1981). I reinforced the flooring by running studs up from the concrete basement floor, and haven't observed any sagging.