I never understood how you can build a car for both roadracing and for drifting. The set ups are very different. Sure, it is possible to drift a roadrace car, and vis versa, but to lay out a plan on a all out, purpose built car that is set up for both is a bit misguided in my book, especially when it comes to suspension as that is where the main differences are.

What are you planning for suspension set up?
What tire set up?
what kind of power are you going to be putting down?

You're going to need a bunch more angle to seriously drift than the stock steering set up will allow.
from what I remember of older mustangs, the rear suspension leaves a lot to be desired. poor geometry and a solid axle are going to make it difficult to control while trying to drift.
road racing, those things can be over come without major modification. But between the steering issues and the rear suspension issues, those are the main reasons why you don't see more mustangs at the drift events.
Is a suspension conversion out of range? would the newer 2005+ style suspension solve these problems?