A NeedsWings single CAI alone will make your automatic run like a 6spd. in the 1/4 mile. (In other words, it will make your car about 1/2 sec. quicker). Doesn't sound like much, but it actually is. Probably the biggest improvement over any other performance mod you can do for the price.
As MrMiata said, nothing short of a V8 swap will ever give you the sound you really want, if you're hoping for anything like the 69 Charger.
By the way, do you have any photo's of your Father-in-laws car? What shade of green is it?