Filler neck relocation

As long as the cap is air/vapor tight and you vent the tank to the outside it will be fine as far as driving fumes. The problem could lie in filling your tank. Gasoline fumes are heavier than air so they will drift down. When you fill your tank the fumes could settle to the lowest parts of your trunk and even travel through the backseat area and settle at the floor of you car.

Chuck