The rakes would just increase drag because you're driving through side winds all day long. In my opinion, don't bother with all sorts of elaborate wing designs. They are like band aids.
The grey one is really beautiful, and I don't say that a lot of 3rd gens. Is it the Iron Duke that came without all the ugly body kits?
If you can raise the rear window that would do fine. Watch the rear sight lines and be aware that you won't be able to smoothly continue the angle of the sides of the rear window. There will have to be surface breaks at the bottom of the window and the top of the quarter panel, and it won't look pretty.
I am sad to say this but maybe using a window from a different car would be the way to go. The Mustang rear window doesn't look too too bad but please don't do anything else that is on that thing.
Source a Trans Am front. It's more aero-friendly and looks cooler. Extend the front surface with a flexible material instead of extending the stock one. You want to keep air from going under the car and the existing air dam probably can't deal well with all the air the nose is pushing down there.
These pace car wheels are PERFECT for an aero modder. If possible, get them! If not, do steelies with screw-on Moon covers.
You lift the back up to template and this thing is a dream.
OK, I have it.
Make a custom louver that is up at the template height. These small steps will do almost as well as a customary Kammback but will look much more period correct. No body mods, no weatherproofing. Paint it black and rock it. It'll look almost stock.