Like was said it may bolt on there but it is not going to look right. 70-72 Darts had a smaller bumper so there is a gap below it that is filled in with a valence. So you are going to have a gap down there and I don't know how easy it would be to add a valence to a set of 73 fenders as they don't have the cut outs and mounting points for it.
The ting to do is change out the whole front end to a 70-72.
Or if you are just trying to loss some weight get a glass 73 bumper.