Improving MPG

four speed manual with a good clutch.

conversion to newer type tranny with lockup converter.

either one is big $$$.

old style 727 or 904 with old school converter = gas down the pipes.
you could get a 9.5 converter from dynamic, but $600 later and install time/$$$ will not se as much improvement as a swap to manual trans.

carb tweaks, improving comp ratio, lowering the weight all help the car also. fan clutch, nippondenso alternator conversion, etc.

however, your real big gotcha is the old style automatic.

best bet: drive it enjoy it for what it is. if you are wanting a commuter car with high mpg get one of the new darts. i get 34.5mpg even though i have my foot in the turbo a lot of the time lol.