I think you are being had. 4.10 gears in an auto might be nice around town, but not good for the highway. If you do decide to go with gears, only buy Ford Racing gear sets and have them installed at the dealer so you can bust them when it falls apart later.
Ask an auto trans
guy who has a tuner AND gears, if he had to choose, which would he rather have, and you will get tuner, hands down.