One gallon of water weighs 8.34 pounds. Multiply by 80 x 8.34 = 667.2 pounds + (the weight of the heater itself). It's only rated for 600 pounds.
Bought this to elevated my mini fridge so it would be off the ground and easier to reach into
It is 16 inches tall. There is a National plumbing code as well as State, County and City codes to which you must adhere to pass inspection. You will have to check with your code enforcement if you plan to have the installation inspected in order to pass. If you don't get it inspected, you will have to address the codes when you sell the home.
My sheet metal gauge read 17 on the legs and 16 gauge on the top.
You could do that by cutting a couple of the legs in half. For safety reasons I would not do that. https://www.ahit.com/news/Garage-Water-Heaters.htm
Can be attached to floor with existing slots on bottom or legs.
Drilling holes in the structural legs may compromise weight capacity of the stand. Not recommended!
The unit does not include brackets for bolting to the floor and they are not needed or recommended. Drilling any holes in the legs could reduce the weight rating which is not something that should do and could result in collapse of the stand and the water heater.