You will need an 8.8.
I'd go 4.10s for the auto, no matter what. I'm doing mine right now. Maybe even 4.30s! Just get some better than stock tires and you will hook fine.
Why do you want an Auburn? I searched for an aftermarket one and couldn't make up my mind because if you look at any board every unit has **** talked about it. They're like: This one sucks. And then someone else is like: No. That one sucks. I had three of those blow up. And then someone else is like: Well ****!
Mustangs come standard with 28-spline axles. Upgrading to a 31 doesn't really do much. If you're going to do all that you might as well just build a 9" and be done with it.
Just get a take-off 8.8, rebuild it, put a girdle on it, and call it a day.