A Monte Carlo bar is steel member bolted between the fenders inside the engine compartment. It lies across the engine bay, just in front of the distributor, and prevents the fenders from sagging inward.
A related support is the export brace, which forms a v between the shock towers and firewall.
For more information about these add-ons, see the Vintage Mustang Forum discussion Export Brace---why?
submitted by a65ragtop