Not every 9" ford rear is bulletproof...
Especially if your refering to OE cases,you better know how to choose the right ones or you could learn about this the hard way.
The folks most concerned with this issue are those looking for every .001-.0001 of ET,and have optimized virtually every other option available to them within the rules of the class they race in.
Say super stock or comp eliminator classes.
The area of concern in stock 9" carriers is the pinion support bearing casting,some of the stock cases are way too thin and too weak to risk in hi-po use,aftermarket cases are far superior to even the OE "N" cases,which are the best OE choice.
And yes strange does offer a drop out 12 bolt centersection to fit a 9" ford rear housing,solves the "3%" issue for those who need to be concerned enough to think about this.
Here are a couple pics to show the weak area for the 9" and such,and some info on the 12 bolt dropout deal.