For 250's, Foxes and AOD's.
The 250 never saw Fox body duty. It's not technically legal in a Stang. It saw service in separte chassis 'Starsky and Hutch' Torninos, X-bodys from the 1969 Falcon/Mustang to the Maverick then through to the last 1980 Granda/Monarch/Versallies. You have a host of minor mods to make.
Check some historical posts by Jack Collins in Dec-Jan 2002. He covered a very cool dual exhast, triple carb 250 into fox Mustang, with a double hump sump to fit the V8 cross member. Stock V8 AOD bits bolt right on the 250. Use the 3.3/200 cross member, and use either FoxGranada or the 200 engine mounts. Bonnet height is tight with the stock 200 engine mounts. 250 engine mounts won't fit the 200 x-member directly without some extra work.
For Big Bell 200's, Foxes and AOD's
The big bell 200 engine, made mostly between 1981 and 1984, will take an AOD if you redrill the transmission in accordance with the Big Bell 200 Trans conversion found in the tech section. The V8 X-member is different, so the 1980 to 1984 Mustang or LTD/ 81 Granada/Cougar 3.3 X-member needs to be used. Optionally, you could fab some mounts so you use the existing V8 x-member.
The flexplate needs to be a 300 I6 item with 164 teeth, or the stock 50 oz AOD 302 item with the weights removed. Then it needs to be redrilled to a 2.75" bolt patern, down from the 3" stock 300/SB Fords ran.
For pre Big Bell 200's, Foxes and AOD's
The earlier pre 81's 200's have small bell housings with the starter at 2 o'clock, rather than a 4 o'clock with the Big Bell 200 and 250 US engines. They won't take an AOD at all unless you get an adaptor made.
I'd like to repeat the words of Big Al. An AOD will most likely suck the life out of a 200 engine.
The stall ratio is between 1650 and 2350 rpm depending on if its an HO trans or not. On a 200 six, the stall speed would be down to the 1200 to 1900 rpm range. The stock AOD torque converter would be hooking up way to early, but the H.O 5.0 converter would be fine on a 200. The AOD is another 75 pounds or so on a C4 or C5. The diff ratio would have to be lowered to a 3.73:1 or so to even allow the car to drive well in the city. Plus is the overdrive would give you a 2.8:1 diff in top.
Technically, a diff ratio change, gearbox change blacklists you for EPA or smog compliance. It's up to you to prove the combo complies.