An AOD won't work anyway. They don't fit.
Find an A4LD from a 90-93 Mustang. You'll need the computer, but it's a plug and play deal. Just swap the computers and you're done. The wiring harness for the automatic is probably already in the car, if not, you'll just need a short pigtail to plug between the harness and the
trans. Really simple stuff if there's a good donor.
The C3 will work and so will a C4 with the right bellhousing. No overdrive and a 3.45 gear ratio may not be horrible, but your son's not going to see the fuel mileage one comes to expect from a 4cyl.