What is the original carburetor of a 71 mustang 351C cobra-jet?

The original carburetor on a 1971 Mustang 351C Cobra Jet was a Holley 780 CFM (cubic feet per minute) four-barrel carburetor. The specific model number often cited is Holley 4180. However, there's a slight chance a different, very similar Holley carb might have been used, depending on the precise build date and factory options. The 4180 is overwhelmingly the most common and correct answer.