I put an AOD into my early Mustang and it fit fine. The issue with size is mostly due to width at the pan and the bottom of the tranny. If you have headers, you may have clearnace problems, but it will fit the tunnel with no problems. Avoid the pre-87 AOD's...they lack an oiling modification that makes the tranny more durable.
The gear Vendors Overdrive may require an additional crossmember or crossmember modifications to support it, and is pretty pricey, IIRC. You'll also have to shorten the driveshaft a good bit. This is no small device either and may not fit into the small portion of the tunnel.
Stick with the AOD. It will fit and is not that hard with the proper parts.
