John, here's a flash picture I just took.
My opening is about 29" wide. The uncut Chevy grille is about 34" wide. The valance was on the car when I bought it. It was broken in the middle, and I threw it in the trash but had 2nd thoughts and pulled it out when I thought I could make something with it. I had a fiberglass repair kit leftover from the days I owned a boat. This might be hard to visualize, but I cut the top piece of it out and glassed it back on to form a 'shelf' along the bottom of the opening. I laid a layer of fiberglass mesh over it which really made it strong. Then I cut about an inch off the bottom in the center. On the bumper, I cut out the section and welded it back on the inside which doubles the thickness and makes the bumper stonger. If you look close you can see the original bottom edge of the bumper along the top inside of the opening. The opening is fairly well boxed-in into the cutout on the body. The bottom front of the trunk is also opened up to allow good airflow into the radiator.
Considering the poor condition of what I had to work with - the bumper, valance, and even the billet grille was far from perfect - I think it came out OK.