I'm assuming this is on a second floor? You want to place the tank in a way that will cross as many joists as possible. You will want it close to a wall (not too close so it doesn't sit on the tack strip under your carpet (I did this once and couldn't figure out why my tank was so crooked). I'm not a structural engineer so I can't comment on how much weight the floor should hold. If the water in the tank moves when you walk by I would consider bracing it.
Edit: Old house = 1987? Mine was built in '57! Luckily I got a nice basement with a concrete slab.