Fiberglass bumpers and brackets for '73 Dusters?

I've been pricing out fiberglass bumpers for my '73 Duster and noticed that some manufacturers sell fiberglass bumper brackets along with the bumpers. What do these brackets do other than save weight? (as in do they place the bumper in the factory location without having to use use the heavy "secondary" bumper?) I also noticed (in the pictures) that the bumpers nor the brackets have any holes drilled in them. Would I have to locate and drill my own holes in a set of glass bumpers and brackets?

If you guys happen to have some pics of the bumpers and brackets out of the box, I would much appreciate being able to see them.

The fiberglass front bumper I got did not come with the holes drilled. You could kinda tell on the bumper where they were suppose to be. So I just drilled one of the top holes and bolted it on and just eyed it up from there and drilled the rest. The fiberglass brackets were just like the factory ones with the holes already there. The bumper and brackets were from WS Racing Fiberglass. The quality of the bumper could have been better. The finish on the bumper was not to my liking so I had to have a friends grandfather refinish it. He also put some fiberglass matting on the back where the bolts would go thru to help re-inforce the bumper.