Larry, You need to know the difference between Engineering numbers,date casting numbers & part numbers
Engineering/Identification numbers on Ford parts show which Carline they were DESIGNED for.
Engines in Mustangs were designed for Main For car lines Like Galaxies ( A) & Fairlanes ( O)
That is why you see C6AE, C6OE numbers on Engine parts
Had that engine been cast SPECIFICALLY for a MUSTANG it would read like this C6ZE-
with the Z denoting a Mustang
The 289 was first designed for a 1964 Large Ford car.....but used in Falcon,Fairlane,Mustang
The Suffix on these numbers denoted Change up's the part has received.
This change can be as minor as a threaded bolt hole to accomodate a bracket
as in C6AE- A , -B -C etc
HTH's