I have a 1989 bronco 351 with a AOD transmission swapped in. I think is has the stock gears in the rear end. At 55 mph in overdrive I am running 1600 rpm and it can feel sluggish if I come across a hill like im gonna burn the od band up so I have refrained from using overdrive unless im going 65+ or on completely flat road. What would be the best gear ratio for this setup ?