I like the rear treatment a lot, not so the front which looks too generic or the sideview, looks akward. Hopefully next gen mustang will be more "avant- garde" than retro.
I think the front would be more successful if it had single headlights and a wider grille. This would make it look more intimidating and keep with the spirit of the original for more design continuity from the current generation. I agree about the awkward side view and think that smaller than 40-inch rims would most likely quell the issue. The hind end would also appear smoother if it was lengthened a foot or two. Overall, though, this is a good base idea for more expansion and improvement.