The 4R70W was a 4 speed automatic with overdrive. It is a descendant of the FMX, and actually very similar to the AOD. Most of the parts even interchange.
There are some big differences though too. It is electronically controlled, which means you have to use a controller to shift your car, and a sensor to tell your transmission the throttle position. That’s the only ‘downside’ to it - if you can even term it that.
It has a lower first and second gear than a C4, C6, FMX, or AOD, which is nice for acceleration. Even in stock form, it handles a lot of power, over 350 horsepower with no changes at all. The engineers did away with the stupid concentric shaft torque converter lockup on the AOD, and went to a very simple ‘on off’ setup. Most controllers only lock up the torque converter in 3rd and 4th gears, but you can often program it to allow lockup in any gear you want - the feature is independent of what gear the transmission is in.
The shift quality is excellent, and because it’s controlled by an external box instead of built-in hydraulic circuits, you can change your transmission’s behavior very easily. For example, many of them have the ability to store more than one tune, so you could have a ‘cruise’ mode where it shifts gently and comfortably, but with the flick of a button, go into ‘sport’ mode, where you have firmer shifts and a tendency to hold RPMs longer under acceleration. Even the cheap controllers are tuneable, so you can make it behave exactly how you want it to.
The hardware upgrades to AOD over the years made later versions a little better, but in the final versions of the 4R70W, they really perfected things, allowing tremendous reliability, superior shift quality, and simplicity.
They were used in a lot of vehicles, but if you want one for your car, I suggest you pick one from 1998-2001, as those will come with all the best upgrades, and still retain the mechanical speedometer provision. The Mustang V6 is a great donor for these, and is basically identical to the V8 version in the years I mentioned. If you use a V6 transmission, chances are it will have no significant wear, even on a high mileage engine.
Be careful to make sure you don’t get one that came off a Mod motor - the bellhousing on a 4R70W is part of the case, and if you don’t get one with a Windsor bolt pattern (which is also what the V6 Mustangs used), it will not bolt up to your smallblock.
It is bigger and heavier than a C4, and uses a little more power. It will also require you to change the transmission crossmember, but any AOD or T5 crossmember designed for your Mustang will fit it fine. You will probably have to shorten your driveshaft. As with any overdrive transmission swap, if you have steeper rear gears, you probably want to go to a lightened driveshaft too - the steel driveshafts tend to flex at high RPMs, which will cause vibration and failures in the differential pinion bearing, u-joints, and transmission tailshaft bushings.
If you can find an Explorer or Mountaineer that has not had its driveshaft crushed with a forklift, you could probably get it shortened for your car without spending very much at all.