2.76 vs 3.23 vs 3.55

As of now, I have a 8 1/4 peg leg rear axle with 2.76 gears. I am thinking of upgrading to 3.23s or 3.55s. I drive mostly around town and a fair amount of time on the freeway. I occasionally make 300 mile trips out to lake Havasu. I have a few questions regarding the new gears.
1) difference in gas mileage between 2.76 and 3.23s AND 3.55s
2) off the line speed between 2.76s (horribly slow lol) and 3.23/3.55s
3) if 3.23s or 3.55s would be more suitable in driving conditions described above.

I drive a 74 dart sport with a 67 318 and a 727.