3.91 good or bad

I'm thinking of running 3.91's in my 70 Swinger. Hooked up behind a TKO 5 speed, should make a very comfortable drive on the highway in 5th gear. Had a TKO in my 69 Camaro and it was a blast to drive, and actually increased the gas mileage by almost double on the highway--in town in lower gears was a different story though! Ben.