Make sure the filler tube is clean on the outside and use grease to slide it all the way into the tank. Sometimes the crud will let gas seep by on the filler tube.

56 Plaza 63 D100 step side 67 Coronet, 68 Roadrunner, 69 Super Bees, 69 Coronet 500 convertible, 70 Roadrunner Post, 79 D150 360, and a severe case of Mopar a,d,d