The OD with the lower gear is the best of both worlds, you get it rolling easier with less throttle input the lower gear, than once at a steady cruising speed on the highway, the OD drops the rpm. Those small block Mopars are very efficient at 1800 -2000 rpm at the chosen mph. Gear the rear end to turn those 1800-2000 rpms at the speed you expect to be driving at the most. Much lower rpm and it works too hard pulling hills of any height and will kill the mpg. Its very hard to find a cam that will produce the torque you need to pull the hills at those low rpms.