I thought I'd update everyone on this. The ducting with the stock IC helped, but temps would still climb when towing up steep inclines. I found a used Wagner IC and bought that.
Wow. I can't believe how the Wagner changed the IAT2! Towing through the Colorado mountains and over to Moab, UT with 102* on the display, I feel like I thoroughly tested the new IC. Temps are now 20-25* above ambient temperature at the IAT2. With the stock IC temps were 50-60* above ambient when towing. The cylinder head temps now climb more slowly. Keeping the truck out of the red is much more manageable now. It goes up hills at 70mph and as long as I drop a gear or two manually, the coolant temps stay out of the red.
I still think the cooling system in these trucks is inadequate. I'd love to stay in 6th gear and keep the RPMs low when towing because I have the power to do so, but I have to downshift to keep the temps low. I'm going to try a pusher fan on the front of the AC condenser first, but eventually will probably upgrade radiators. BTW, I have the Max Tow radiator, which is the largest stock one.
The added power of the Wagner IC is nice too.