Regular 1970 non-ECS cars should just have the one tube for the gas filler.

ECS cars, which are 1970 CA only, have the gas fill tube and the smaller vent tube in the trunk that has several rubber hoses that run from the nipples on the side of the gas tank up into the vent tube.

If you car is was not sold new in CA, and does not have the smaller vent tube in the trunk, then it's a non-ECS car and can use the regular fuel tank.

https://board.moparts.org/ubbthreads/ubbthreads.php/topics/2432466/70-cuda-ecs.html

https://www.allpar.com/threads/fixing-1970s-and-1980s-fuel-evaporation-control-systems.229057/