An export brace is a v-shaped steel member bolted into the engine compartment to stiffen the Mustang body. The point of the v is bolted to the firewall, and the ends are bolted to the shock-absorber tower caps.
Do you need one?
Well, Ford engineers didn't think so when the Mustang was new. The "export" in the name means it wasn't put on cars sold in the U.S. But now, 35-40 years later, owners who have put them on their cars say they feel a definite improvement in handling, especially when cornering.
A related brace is the Monte Carlo bar, which lays across the engine bay, bolted between the fenders just in front of the distributor.
For more information about these add-ons, see the Vintage Mustang FAQ topic What is a Monte Carlo bar?
submitted by a65ragtop