My 2007 MegaCab has the 3.73 gears. I would think 3.42 would probably cause too low an rpm when towing at highway speeds and you'd inevitably be running a lower gear to get the rpm into a better range. At 70 mph I'm only running 1750 rpm and at 60 it's about 1600. That's in sixth gear.
I just checked
www.dodge.com . If I'm reading it correctly, the 3.42 is standard on the 2500 and the 3.73 on the 3500. You might try the build your own and see what comes up. I didn't go that far. It may be that on the 2500 the 3.42 is standard but if you select the tow package it adds the 3.73. But I didn't check into that.