I think everyone here has overlooked the cheapest but best mod for an automatic.....A transmission cooler! I say buy a transmission cooler, a set of 4.10 or 4.30 gears, and when you have enough money, get the programmer then or better yet, get a dyno tune.
Buy the gears at
www.stangstore.com or from Randy Stinchcomb at Liberty Ford out of Maryland. You can contact him over at Modular Depot as well.