I had a 2016 Ram 3500 SRW with the 3.42 (only ratio available). I just traded it for a 2018 Ram dually with Aisin and 3.73. I got the 3.73 for towing the 5th wheel. I think you will be fine with a 3.42. If you get in hills you could always lock out 6th gear if it's shifting too much. I experimented towing the 5th through Smokey Mtns., and left it in 6th gear, let it shift a lot, and never saw a change in transmission temp.
You're good to go......
add: the front Torklift tiedowns are so much easier to install on the 2018. No fish wire!! But get you a small pair of needle nose vice grips to hold the back-up plate while you start the bolt in the welded on nut. It'll make more sense when you see it.
2018 Ram 3500 CC LB DRW 4X4 Cummins Aisin Laramie Pearl White
2018 Landmark Oshkosh
2008 Bigfoot 25C9.4
2014 NauticStar 21 ShallowBay 150HP Yamaha
2016 GoDevil 18X44 35HP Surface Drive