There are a couple of other automatic options. Silver Sport Transmissions has adapted a GM 4L60E 4 speed overdrive automatic transmission for Mopars. There is no cutting or modifications required. See: http://shiftsst.com/dart-valiant-barracuda-duster-demon-1964-1976#a41-automatic
TCI also offers a similar automatic that is higher horsepower capable with 6 forward speeds: http://www.tciauto.com/tc/transmission-packageshtml
There are some other potential manual options, but by far the best in my opinion is the Passon 5 speed.