Identify Disc Brakes on Dart?

They sure look like the 73+ disc brake set up from an a body. But there are other factory set ups that will also work. Yes the disc brakes did increase the track width front to rear with the front being wider. Should be the 10.87” diameter rotor.
Here’s an article that may help you further. It does state that the 73+ B/R and F/J/M spindles cause geometry issues. I’ve not personally used these but others here have said they have so might want to search FABO.

DISC-O-TECH: Stop on a dime from Mopar Action and Rick Ehrenberg