While the 3.42 is rated to pull XXXX I would personslly go with a 3.72 or possibly a 4.10. My 07 dually came with the 4.10, and honestly I would not have anything different. I was easily able to tow my 13,500 pound fiver all day with the cruise control on. At 70MPH towing I was right at 1850 RPM which was the middle low end of the torque curve. Right where I wanted to be. Stomp down on the go pedal and the truck would pull like a freight train. To a hundred. Did I sacrifice MPG? Yea, maybe 1 or two empty. But at 17 MPG at 70 empty and 10.5-11.5 towing I was totally satisfied. If you want the 4.10 then you need to opt for the Aisin, which as I understand is not a bad thing. Dont get me wrong the 68RFE has been proven a strong reliable trans too. Its all dependent on you. But if your going dually, go big or go home!