If the pump you run has one line then plug the return on the tank as well as the third(vent)just be sure to run a vented cap.
If your pump has 2 lines I would change it to the single line,
I am not sure when evap systems were implemented in cars, somewhere in the 70s ,The sender with the 3 lines is for a vehicle with a return style fuel pump and an evaporative system.The single is for a straight shot to the pump, no return and no evap.It needs a vented cap, or your system will starve for fuel as vacuum builds in the tank.
I have seen the return style pumps have issues with pressure to the carb in the past, that is why I said to run the single line setup.Less chance of failure
I think the return setup was to help keep cooler fuel to the engine by recirculating it back to the tank, supposedly helping to keep vapor lock to a minimum.Like I said ,IIRC it only causes low pressure issues and delivery volume to the carb as well.
Also it sounds like your car only has one line from the tank to the front, r did I read that wrong?