3.91 gears for shorter 60 mph trips

Too Low for me:
I have a 340 6pk 4spd 73 cuda with 3:90's 29" tire. It is fun to drive, but I wouldn't leave town and evade freeways as much as possible.
My wife has a 71 Dart with 360 2500 Hughes converter, 28" tire, and 3:55's it is just as bad. Everybody was right on with the 3000 RPM's @ 60 MPH.Her car has solid lifters, and it's very noisy even with quite mufflers.