That's the Monte Carlo brace Glen is pointing at (also recomended), not the export brace. The export brace does not need extra holes. Simply remove the two top bolts (each side) for the shocks, the three nuts for the shock's braces, the old firewall braces and "fit" the export brace. All the old hardware is re-usable. On a couple of my cars, I use a large drift pin to align the brace at the firewall.
If you are considering showing your Mustang, don't drill the extra holes for the Monte Carlo brace, the brace will be just as functional. And use a 5/16-18x1.0 HH bolt without a washer under the bolt's head. That also applies to the existing bolt. Too tall a profile of the bolts will rub on the under side of your hood.