Many years ago, I swapped the 3.23 gears out of my 64 300 for 4.10's. Mind you this was a mild 413 with the street hemi grind cam in it and a three speed stick. Highway mileage jumped from 12 to 15 mpg. Very consistently too.
Being in the cam's sweet spot is the key, as mentioned. Odds are you will want to look in the small cam part of the catalog, not need for big lift or duration numbers. Don't forget today's cars are optimized in regards to friction reduction. Roller cams, no drag calipers, etc.
They say there are no such thing as a stupid question. They say there is always the exception that proves the rule. Don't be the exception.
