4.10's aren't overkill for an automatic. The majority of people I see with auto's generally go with 4.10's and love them.why would you say that? how are gears overkill, rule of thumb is subframeconnectors and gears should be the first modifications to a 5.0 mustang. and also the best gear choice for an auto is 4.10, manual is 3.73. sou why are they overkill?
After gears and SFC's, then I would STRONGLY suggest a good shift kit. Then your mustang will really drive like a Mustang!