Slant 6 most mpg's

Your driving style will also have a lot to do with it.
Basically, you must drive it very methodically with no extra revs between gears and no jack rabbit starts,
In other words, drive like an old man and stick to just under the speed limit with only enough throttle opening to get the job done...........
It might help to install a vacuum gauge and drive the car with the highest vacuum possible at all times.
Having the tallest, skinniest, smoothest tread, hardest tire that you can fit on your rims helps with a low restriction exhaust
will help as well.
A great engine tune (as previously discussed) is paramount and if you could lighten the car further that always helps too.
Finally, as stupid as it sounds, make sure you buy good gas to run from a reputable station!