It's gonna take a *lot* of sawsall work to get a 4200 lb T-Bird significantly below 3000 lbs. Are carbon fiber or fiberglass body panels allowed? You'd probably have to do the hood, trunk, all four fenders, and section the roof in c/f to get that thing much lighter.
Good points on the suspension. It's far more SVT cobra than GT mustang, though I thought the T-Bird had a McPherson up front, but I really can't remember since I never did anything to my old T-Bird's suspension (except bitch about how soft and worn out it was).
The MN12 body is pretty slick... and that car is one of the last NASCAR platforms that was actually faithful to it's racing counterpart, so it probably does alright aero-wise. Though they certainly need that front splitter, as I remember my T-Bird getting *very* light in any sort of cross breeze once the speedo's north of 90 mph. Too much air under that car.