When i first read you comment, i thought to myself that you were wrong. I have had one 1974 dart sport, and one 1974 swinger. I am pretty sure that they both had an inset grill, but then I was thinking about it more, and I think the swinger had a grill the same as a 1973, but as you can see in the pic of the brown dart sport I had, it has an inset grill, and it was a 1974. Go figure.
and yes you are right about getting the header, that would fix the mounting holes not lining up.
Phil