Which gas tank?

Okay, I am confused. I did find on ebay "spectra" tanks.
For my 71 Valiant, it does show early/late model.
So 4 vents on the side of the tank "early model",
One vent at the top above the sending unit "late model".
Here's where my problem lays,.
My 1971 Valiant has a used tank in it, that the last guy put in.
It is out of a 69 Dart he told me. Which makes sense.
Okay, there is "no" vent tube on it at all! So I have nothing to go by?
I have no idea which tank to get.
If I get the one with the "one vent tube" at the top of the tank.
Do I just put a rubber vacuum plug on that vent tube? Will that build pressure if not vented?
I just want to make sure. I am ready to order my tank!