Although some people like 4.10s with a manual, most don't. I have had them, and I didn't. I would look at 3.55s or 3.73s. I have had both of those as well. I currently have the 3.73s and I love them! The 3.55s were a big improvement over stock and might offer a little better MPG, but the 3.73s make it a bit more lively.
Around town I pull 16-17 MPG with the 3.73s, last year I did an extended road trip and managed 21.6 on the highway.
Whichever gear you get you will love them. The stock gears are ridiculous. This is a handy calculator: Ford Mustang: Mustang Tech: Calculators & Decoders: RPM Calculator