Im doing a similar conversion for my 66 mustang w/302. I talked to the guys at Art Carr Transmissions and they told me what was the best deal for AODs. Basically, the later the better; but if you can, try to get one from 86-93 instead of the earlier ones. These years had the more desireable throttle valve cable hookups, and although the earlier ones can be converted through drilling, they sell more conversion parts for the later style. One is a lever and one is a shaft, i can't remember which is which though.
Also, the main weakness in the AOD transmission is the two input shafts(one inside the other). Ford did this to lock up in 4th gear for 0 torque multiplication, or better gas mileage. These input shafts tend to shatter under higher power sending pieces through the whole tranny. Now, depending on if you are leaning more toward the economy or performance side of the spectrum, i wouldn't worry about it. If you are going more toward the performance side, then I would strongly advise getting a single hardened input shaft for non-lockup usage. Doing this alone without touching anything else in the tranny can account for 40hp at the wheels. Also, torque converter selection is greater of the non-lockup variety. The second weak point is the Overdrive Band. You can get wider OD bands from any reputable tranny shop.
Here is a site devoted specifically to late model mustang drivetrain parts into early mustangs <a href="http://www.windsor-fox.com" target="_blank">www.windsor-fox.com</a>
Here is a site about a kid who put an AOD into a 72 mustang w/351. <a href="http://www.mustangsandmore.com/ubb/DanJonesC4AODswap.html" target="_blank">www.mustangsandmore.com/ubb/DanJonesC4AODswap.html</a>
The good thing about this swap is that it is advantageous for economy without hurting performance and it HAS been done before.