What you need is someone with a Dart Sport to take some measurements for you!
When I first saw this thread my first thought was "nope, not gonna work". But after looking at a couple pictures, and then superimposing a picture of a Dart Sport tail over a picture of my car when it had the '74 Duster tail, it's really close. Obviously I'm not gonna find another picture to match mine perfectly, but looking at the tail light buckets online I wouldn't be surprised if they fit.
The problem you're going to have is that Dart Sport tail light housings, bezels and lenses aren't reproduced. And they're pretty expensive, at least on eBay. Ultimately I think the only way you're going to know for sure is to test fit a Dart Sport tail light bucket, which means you're going to have to buy one.