I have done a heap of research on body reinforcing and welding that may have been done at Holman Moody and... I can't see in any period pictures or anywhere of any reinforcing apart from when the cars were built to group 5 and there was extra reinforcing added around the rear spring hanger chassis holes as some cracking occurred.... after 1964 and then 2 years of full on racing, they went to wider rims, more traction, more stress on the suspension, this is likely what caused the stress cracks.
Of course safety being what it wasn't in 1964 there were no roll cages in the 64' Monte Carlo Falcons.
The Falcons did not have the upper Monte Carlo rally bar above the engine, only mustangs were fitted with these.
The MCR falcons had a lower suspension brace bar likely H-M fabrication that fitted under the sump and attached to the engine bracket bolts. A very tight fit. Also had the export brace fitted to the upper front suspension towers.
Both 63 & 64 had long aluminium skid plates under the of the front engine and these were bolted to the front chassis and engine brace. Also in rear below the fuel tank there was another skid plate
Below is the 6 cylinder lower brace bar, this does not fit with the V8 engine mount configuration and quick ratio steering.
