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With a 3.5 no problem at all. The 2.7 should be well within specs, but you'll notice the weight. Still shouldn't be a problem compared to what we were all towing with just a few years back.
Just my personal opinion here. I would seriously consider the 3.55 gears over the 3.31. I did a back to back test drive (of a 3.5 EB) and for me the 3.31 had less power as I started out from a stop. The 3.55 was noticably stronger. Drive both and report back here if you can.
If he's not towing often he may well do better with the 3.31 gears for overall economy and wear and tear. Not everyone wants to get out of the hole like a rocket.
I drove trucks with 3.31 and 3.55 gears, and ended up buying one with 3.55s simply because that's where the deal was. I would have preferred 3.31s.
These trucks have far more than enough power to tow anything within, or even substantially over, the rated towing capacity. My '13 with 3.73s would spin the tires with 12,000 lbs behind it and enough tongue weight to load the axle to GAWR. I want fuel economy and low cost of operation more than I want to be the fastest up the on-ramp with a trailer. I like my truck, but I'd like it more with lower gears.
Will only be towing maybe 3 times a year. then it will be either to Fla. or AZ. From the MIlwaukee area. Would like the better gas mpg from the lower gear ratio.