Gas Sending Unit Tubes- 2 Questions

[Zonaman]

My Gas sending unit has quit on me. The gauge goes up when disconnected and drops to E when connected. A float perhaps? In the event I need to change the entire unit I noticed that my '69 has a two-tube unit on it looks like either a vent or some kind of return. I though that was only on the '72? Will it make a difference if I use the two tube as it is now or should it be a single line setup?

Thanks.