Put a Gear Vendors unit behind your C4 three-speed automatic and you’ve got a car with six forward speeds and two in reverse. I don't know the price on these kits, but they work great and use your original transmission.
http://www.mustangandfords.com/howto/38418/
This past summer I ran 3 legs of Hot Rod magazines "Power Tour". I ended up taking my 92 Mark VII LSC Special Edition, cause with a 3 speed auto and 3:55 gears my pony is not happy on the highway. After meeting people with older muscle cars and asking about theiir auto trannies, I found alot of people were running these overdrives with their original trannies. They chose this instead of more modern applications. It was cool to see some real hopped up muscle cars driving 70 MPH near me on the highway, then we would stop at local drag ways, and they would run down the strip with respectable times.
Just a option...
I saw a link to a site about doing a AOD conversion, can't find it now. One of the most important things they stated was getting brackets and misc. from the donor car, to ease the swap. Either way, keep us informed of your progress.