I think it is best to either drain it all the way and leave the lid off or fill it up all the way and plug the vents to help keep condensation out
That is what I have heard from other racers with roundy rounder cars with big cells
One other thing to consider is to remove the race gas or pump gas and go to your local airport and buy enough 100 LL avaition fuel to fill the cell up, avaition fuel is designed to have a 18 month shelf life and still work okay in airplanes with fuel cells or aluminum fuel tanks