I ended up going with a 5.0 on my way home from work last week. At the time I was completely stock(only had the car 2 weeks now) and the 5.0 couldnt get away. He would pull but not by much maybe a car, car and a half. Keep in mind these were highway speeds going from 65+ if it were from 40 it would be even closer since the weight plays a bigger roll the slower your moving.
I now have jb4 running e85/93 mix and with how much harder the car pulls I think I would have pulled quite hard that mustang. you are just about FBO, a stock 5.0 with exhaust should be a total cakewalk for you.