I cut my core support today for in preparation for the HHR install. My radiator is and AFCO, and the welded mounting flanges on it set it back so far that I only had 2-7/8" of clearance.

I thought it might be a little easier to see what modification is needed with the radiator out. Since the mounts are pretty far forward on the sides of the radiator, I had to fab up some 1/2" spacers out of aluminum tubing to bring it forward, and narrow the side mounts on the rad about 1" so I could slid it in from the top. I also had to cut slots at the edges to allow the flanges to clear the core support on the sides...