Should be able to get shipping weight from a retailer online and then each gal of water weighs about 8lbs.
Also note that the rock and substrate is going to displace some water, but you could go for the high end and just add them all together so you know you're safe. You're probably looking at close to 800lbs in the tank's footprint, but that's some serious guessing.
I feel you, I just put a 40gal on the 2nd floor apartment I live in and I'm still listening for creeks and thumps in the floors.
It should be enough. Seriously don't quote me on this but somewhere in life I "picked up" the info that each floor has to be able to hold 1000lbs or it's not "up-to-code" or something or rather. It kinda makes sense tho. 2 Full grown adults on a king sized bed set can weigh around 1000lbs, or even a large sofa with 3 full grown adults can weigh over 800lbs easily.