It was the set-up the Trans Am Mustangs were intended to use in 1969. The SCCA inspectors saw the set-up and changed the rules making dual carburetors illegal. They are the only Holley carburetors ever made for IR "out of the box".
The carbs are actually under-sized for a 351 ... but they're bigger than 48mm Webers. They are popular with Mopar guys, they're even more undersized for a 426 Hemi!