diff swap

8.8" will take 400rwhp no problem...be far less expensive to mod and rebuild than fix up and upgrade a 9"...although 9" gets cool points, it also saps a little more power from your drivetrain.

The only things you need to hook up an 8.8" is a new center (cheap), 31 spline axles, weld the tubes, put on a girdle and you'd be gtg for the power level you want.

...JMO

Control Arm mounts need to be welded on to properly mount a 9" in a fox, it is not a straight forward swap, you need new control arms, new brakes...

Cheaper and much easier to build a 8.8 or have one built. I do a pretty much bullet proof 8.8, it features 9" bearing ends, 31 spline Moser Axles (4 or 5 lug), 31 spline Eaton Diff, all new bearings, welded tubes and brackets, and support braces, girdle, gear of your choice $1750.00 (housing not included in that price)

You can rebuild the stock diff cheap as well if your not ready to upgrade. I have a customer that may be selling his 28 spline Moser set-up, everything is brand new if your intrested.

What he said