I have a 1929 Nash touring with a 5.23 axle ratio. Engine speed gets to red line at 45 mph. Would like to travel at highway speeds (55-60)Would like to add standalone overdrive in center of vehicle. This would require 2 open drive shafts . One at each end of overdrive. An article I read sometime n the past talked about a Volvo overdrive unit being used for that purpose.
Can anyone enlighten me on the subject.

Thanks
al ruckey
michigan