On a Dart Sport Maybe not the Duster. It's the Cars that came with the impact bumper( piston supports) and the filler panel that pushes the bumper out about 3 inches. All 75's and 76's had them.
FYI, my '74 has the 5mph bumper front and back. From the factory.
But I have a friend that had a '74 as well and had the standard bumper on the front. Can't remember if the rear had a standard or 5mph one or not, but I know the front wasn't for sure.