Question about gas tank vents 4 vs. 1

This is a good topic. My 72 Duster has one return line going into the top of the gas tank and it goes all the way up to the motor, Where I believe it is suppose to tie into the charcoal cannister. no lines going to the gas fill tube at all. This is originally a 340 car. My dilehma is my new 3/8 sending unit I bought also has another vent tube coming out that i won't need to use. it must be for the cars with mulitiple vent tubes, would be my guess. somebody with a little more knowledge on this will hopefully chime in.