I'm not sure I understand just what you are doing, but if you have a 9500 series with a 671 with a 5 and 2, you don't need any more gears. If it doesn't go fast enough, faster 2 speeds are hard to find. I would recommend going to a single speed axle out of a modern truck with a 10 speed transmission. You can then get the exact ratios you want. How fast are you planning on going. We used to gear Detroits to run the desired speed at 21 or 2200 and then set the governor at about 2400. Don't ever let it get below 1800.