I have an '06 2500 4x4 quad cab short box with the Hemi and 6 speed manual. I have owned six other 3rd gen trucks, one other Hemi, the rest CTD's.
The 5.7 is a complete pooch in the mountains, worthless and gutless with any weight behind it. Now this is at altitude, from 7000 feet up to 11,000. Still, I was down to third gear, which is more like second with the granny first, doing 30 in a 60mph zone trying to pull an empty flatbed car trailer up the pass. My CTD work truck will tow 25k up the same hill better than the hemi...
I also had an '03 Hemi automatic. You DEFINATELY don't want one of those. The 2-3 gear spacing is horrendous for towing on the 5 speed auto.
The problem is the high torque peaks in the newer gas V8s. My Hemi has a max torque at 4200 rpm, that is caning it pretty hard to make power.
If you can stomach the gas bill, a GM with the 8100 is a great puller as is the '05 and newer Ford V10. That said, my Hemi Dodge is a 12-13mpg truck, that's my highway mileage.
I wouldn't do it... I will sell you mine though, 140k miles, $11k
