Transmission Swap (Detailed Question)

[2001GTPEWTER]

I'd like to stop by but NY and NC (unfortunately) are'nt neighbors..I appreciate the offer though. The reason I want to convert this car after any and all engine work is done is because I drive the car every day, and for two reasons an AOD or AODE will be better right now: I enjoy the 5-speed Ford tranny but I have a tendency to beat on it, its' hard to drive a car like this slowely every morning on the way to work. Also I think having a more controlled gear-shifting pattern will save my gas, which is forever too high. I know, cars like this should never have automatic transmissions...but to at least tell people I put it in myself will save a little of my pride. Will I need a new drive shaft? How about new crossmembers? I see aftermarket AODs' that are designed for specific Mustangs and prorated for racing with the promise of smooth everyday driving...are these a better idea than going to Ford? Lastly, most importantly, will doing this the right way in any way harm the car over a period of time?