why a Ford 9"?

Yup, people use them because they are generally cheaper and abundant. I won't.

The 8.75" rear is stronger than the ford 9" in stock form and, contrary to popular belief, the 9" is a copy of the 8.75"! There is a ton of aftermarket stuff for them to make them better. Most of that stuff is not required for a 8.75". How strong is a 8.75"? The TF727 equipped 68 SS Hemi Barracudas had them and ran mid to low 10's in 68. They are currently sub 9 second cars!
You can even get an aluminum case for the 8.75" now. Very nice and even stronger than the iron case. $$$$$$$$$$$$:headbang: