What options for O/D behind a S/6?

IMHO i have a big 4 barrel (390 dual feed double pumper) with headers on my 225 slant and i get 20 to 22mpg hwy running premium(93) in mine.(ethanol in modern gas affects mileage big time) the biggest thing is tuning the carb and timing just right. once thats done the 904 and a stock slant can easily get close to 30mpg. if you want the best fuel economy put a 4spd behind it.

Set the gap larger on the plugs
tune the carb as best as possible
3.23 gears (best accel and mileage combo)
free up the exhaust
adjust the valves
pertronix points to electronic conversion
decent ignition coil.

All in about 200 unless you put a different rear end in it.