I have the FMX also, but when I looked at the Gear Vendor, there wasn't an automatic mode. Do you know how they accomplish that? Something to think about is the need to make the driveshaft significantly shorter and how that affects driveline angles and vibrations.
Instead of an AOD I intend to go with a 4R70W, which is the last version of the AOD that improved all its shortcomings. It requires an electronic transmission control and, if you're handy, it is a bolt-in conversion, but the driveshaft does need to be shortened somewhat. There is a guy that makes transmission supports and a shifter mod that allows the use of the stock shifter. If you're interested I'll find his contact information.
Heres a thread on the 4R70W;
https://www.vintage-mustang.com/threads/1965-4r70w-conversion-complete-how-to-pic-heavy.1119498/page-8