IMO, this is a good stock setup:
17 or 18x 9 wheels all the way around with 245 tires
HR, Steeda or other name brand lowering springs (if you use Steeda use the "street" springs) I spent a lot of time getting the car level, refrain from adding any rake if possible, this just adds more weight on the nose, which Mustangs don't need (I believe in soft springs/ heavy sway bars, your results may differ) In the other car I searched around and found a decent combination of springs from several years mustangs.
Steeda tall ball joints for the front (absolutely mandatory, these actually made more difference than anything else)
Bump steer links
I have to use mostly stock parts so I used the convertible subframe connectors and the vert , K frame brace for the front along with an export brace (you newbs will call this a shock tower brace)
I also have to use the stock upper shock mounts, you probably shouldn't
I use a decent brand shock like Tokico, but I use adjustable shocks on the rear for tuning
A GT front sway bar and hunt around for a rear bar out of a mid 80s Fairmont wagon (Ford part)
I ditched the goofy horizontal shocks.
In some cases I use a piece of cable on each side to limit body lift (an old 60's trans am trick)

Also, you need the "right" car, I searched around for a V6, 5 speed Mustang to autocross because they use aluminum heads (light),, factory headers (light), and have 2 cylinders less weight on the nose. Since I'm restricted to mostly stock parts I had to be creative. I have no problem hanging with the v8s and other stuff, even at Grassroots 2010 we bested all the other muscle cars (and most of the v8 transplants)that ran.

This should get you in the ball park for a "Stock" class car.