Difference in trans = better milage

Hello, I don't know if this will be considered a performance issue but I have a 1971 340 Dart with a 904 trans 8 3/4 diff 2.76 ratio with small block pattern lugs. I drive it on the freeway doing 70 mph at about 3k rpm's. If I was to put in a 727 auto, would that up the mph some. I was told that the 727 auto has a higher final gear drive that makes the car drive faster with less rpm's. Thanks for all your help on this matter