Lug size on a 72 Dart?

If you have left hand thread studs on it, it will have a "L" on the end of the stud, and be 7/16x20 LHT. If you still have left hand threads, you will have to change the studs to RHT to use Cragars, because there are no, none, zero, left hand thread conical shoulder lugs.
There ARE (or, more properly,were) left hand thread mag lugs, I have some, but they are not available anywhere anymore.
If you go with aftermarket rallyes, you probably can keep your studs, and use the lugnuts you have.