In the pics above, you will see circled in red the places that need to be trimmed on the back side of the bumper to clear the bumper support. Also is the large opening in front. There is normally a bar that goes across that opening. (see below) I cut that out to open it up for better airflow to the IC and radiator. It does need to be trimmed back to clear the bumper support as well. Just go slow. Trim a little and re-test fit, etc, until you are happy.
I didn't drill new holes on the bumper cover. I drilled them in the bumper support. I used sheet metal screws with washers under them to secure the bumper. The only stock hole in the bumper support i used is the one in the middle. Now when you go to attach the bumper to the fenders, if your stock bolts didn't break taking them out, you are in good shape. The Impreza bumper has oblong holes for mounting, and will work with the stock bolt. If your bolts broke, grab your drill, and go to town drilling out the broken bolt. You can retap it after its out, or just go with a larger bit so you can use a bolt and nut.
Both my stock bolts broke, so i found it a good time to stick on my Z3 fenders. I lined up the bumper, drilled a hole in the fender and bolted it up.
Here are some side pics of the "finished" install.
