My experience was with the Challengers, I was looking in '73, wound up buying a '74 model.
I think the bumpers for all models (all mfgs for that matter) were that the 5 MPH requirement kicked in for the front in '73, then was extended to the rear in '74.
In the E-bodies, the 340 was the HP option in '73, but they went to the 360 in '74. I think I've seen '74 Darts with the 360, I'd have to defer to someone with more A-body experience for the '73 options.