Sorry guys, it's neither an 8 or 9 incher. It's that odd ball 9-3/8 that Ford used on some pick ups and cars with 429/460s and Lincolns

See the very first photo here: Kevinstang's Ford Nine Inch Differential Page +

Supposedly a 9" center section will fit though never tried it.

(The guys at Hotrodders.com didn't know either)