Gas over flow

I have a 1968 Corvette which has some kind of gas over flow problem when the tank is full. I found out about it after filling up one day and I was told they could see gas spilling from under the car, when I accelerated. The guy said he had a Vett like mine with the same problem but we were at a traffic light and he couldn't go into detail. When the car is parked in my gaurage, it makes the space smell like a speed shop full of gas and oil. I have a new crate motor in it with less then three hundred miles on it. Has anyone else had this kind of problem? I was thinking about plugging up the air vent at the filler cap but that seems wrong as it was put there for a reason.