If the building was built in the 70's and there is no obvious reason to think that it's not up to code (i.e. it looks like an permitted addition, etc.) then you should be OK. Of course, you need to take a good look at the area where the tank will be and see if there is any evidence of water damage or some other factor that would weaken the floor, and use common sense.
If everything looks ok, there is no reason the floor shouldn't be able to support a 500+ pound dead weight. It's important to look at the surface that will actual bear the weight points, as mentioned above. But the, a well balance stand will put around 150+ pounds on each of 4 legs so there is a good chance you'll be ok. Having a carpet with plywood under it will distribute the weight nicely.
So, I'm no engineer (seriously not) but I looked into this when I was planning own tank location. My floor is 120 years old and has some deficiencies, so I put a pipejack in the basement just to help out, but that was a special situation
