I have a '67 and its the same deal. Around here, for SOME reason, they've stopped putting the big rubber cones around the end at newer stations, and it helps alot.
I hold the pump almost upside down, roughly 45 degrees clockwise from upside down. If you look at the contour of the fuel filler stick it in as far as you can following that contour, and then turn it back counter clockwise 45-60 degrees. Am I painting a good picture? Start slow and see how much you can crank the speed up.
With the new pumps I can pump it full speed. The ones with the cones I can usually go 1/3 - 1/2.
Be patient and you'll fill it. Its the design of the car and the new pumps. Everyone with a 66/67 Plymouth has the same problem, its not something wrong with your vent system. I've gotten much better at it over the years.