Replacing sending unit- how low does the tank need to be???

Also, my house is connected to my 2 car garage and I will confirm. IT SUCKS. Can't paint, use any cleaner, or run the cars without the house smelling awful. I daily drove my dart with the slant 6 and kept it in the garage at night. I had to roll it out in the colder months before starting it since the old carb ran excessively rich with the choke on. I could fire it up in warm weather and drive away without issue. But the winter sucked.