Take filler out of car,
Measure inside of filler neck,
Measure outside of unleaded fuel pump nozzle,
Find or make a washer that fits inside your filler neck and will fit around the pump nozzle,
Tack weld this washer into your filler neck,
Reattach filler to car.
Remember to tack weld the washer in far enough down so that you can still screw in the filler cap. If you cant get the washer in easily, cut the filler neck, weld the washer in then weld the filler neck back together.