In other words, and a shaky memory, remove the sender, place the o-ring in its location on the outside of the tank, then replace the sender. Therefore it doesn't have to be stretched over the base.
With the tank upside down on something soft to protect the paint, if you feel any resistance when installing the sender, stop. Look to make sure the float isn't on the wrong side of the drain pipes or other obstructions. Make sure the o-ring contact surfaces are clean and smooth.