331 is a great idea, check out the motor's at dssracing.com....that is where alot of my friend's buy there engines.
Your gear selection's are good for a stock street car.....if you decide to keep the car n/a and maybe run nitrous, look into the 3.90's......if you have plan's for a blower or turbos, look at 3.55's.......
hotwheels of insanemustangs.com