If it has an Eaton 2 speed, different rears that interchange, but there were no fast ratios in that period. If you are putting in a 396 you could pull faster gears. This can be accomplished by either using a 5 speed overdrive or a complete rear end out of a late model truck. Our over the road trucks have the high twos with a direct and the low threes with an od transmission. However their torque rating is at 1200 rpm and we have the governor set at 1700 rpm. These engines weigh 4000 pounds and simply won't work in your application. You can use this calculator to determine your road speed. http://www.free-online-calculator.c...d_calculator/road_speed_calculator.shtml