Gas tank for 1970 dart

The first one is the correct one for your car, but it's inferior as it has no vent.

The 2nd one is a definitely no-no, that's the 4-vent emissions style in 71, commonly known as CA emissions.

If you want the best direct-fit gas tank get the 72+ tank like the Spectra CR11E.
What you're looking for is the tank with the single vent line in the top-middle facing forward, above the sending unit. Best vent design of the stock tanks.