Cant put gas in new tank. Help

My 66 Valiant is the same way. I can only get 4 gallons in it from empty out of a gas can then it comes out as if the tank is full, all over my quarter panel. I think that my filler tube is to long and once gas gets up over it it will not let any more in as the tank itself has no vent. Just the vent tube way up top.
I plan on pulling my tube out and checking, I just havent got to it yet.
Paul