5 speed trans. Let the trans do some of the work. And driving style to a large extent. Make sure everything is tuned right. Electronic ignition will help driveability as much if not more the fuel economy. Higher gear (lower number) helps highway economy. Taller tires have the same effect.
Yeah 5 speed tranny or AOD would help, but mainly on the highway. Getting 16 or 17 wheels would help, especially if it had 14's. If it's got a 4 barrel, make sure it's staged to where only the back 2 open when you punch it. MPG is certainly not a reason to get one, the reason is that they are simply badass.