There's not a "fill limiter valve" as such on these cars. The tank plumbing is designed with a restricter in the vent line which causes the fuel to back up into the fill tube when the tank is nearly full. It is possible to get about two more gallons in there after the gas nozzle shuts off if you dribble it in very slowly. GM did this to provide vapor space in the tank, but I've never had any problems with filling it to the brim.
Rod Schneider, Woodstock, Ga.
"You can't have too many toys!"
1988 Fiero GT
1966 Porsche 911
Van's RV-6 airplane-under construction