Actually, no and no.
There is nothing such as an unvented gas tank for a '66 Dart. The CR11A is a direct replacement for the original tank, which is vented by means of a 3/16" steel line that comes out the top of the tank and runs up the left-front corner of the trunk compartment, makes a sharp U-turn and heads back down, through the trunk floorpan, and the open end is not far from the tailpipe. You can see a portion of the vent line in the first pic (click to enlarge) on
this page (he calls it the "vapor line").
A vented gas cap is not required nor helpful on a car like yours, which was not designed to use a vented cap, and there isn't one that will readily fit. If you cram or adapt one on anyhow, you will worsen the gas-down-the-side-of-the-car problem, not improve it.
Make sure the face of your filler neck (where the gas cap seals) is smooth and not dented, scarred, or otherwise defective. Get one of the good-sealing caps from
@damraider on here, make sure your fuel tank vent is properly installed, properly routed, and not blocked, and you'll be good to go.