You may be over filling it. You should stop putting gas in when it kicks off. Old cars dont have extra room for fuel expansion. Gas is cool when under ground and expands went it a warm area like on a hot day or by hot exhaust and it will come out the tank. My 69 Camper Special used to do it all the time.

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1964 Galaxie
1967 Cougar
1968 F250 Camper Special