A caveat, the turbo mouthpiece should be done by everyone that owns a LLY, the other things are for people that tow and have a heat issue.
A couple things. As RickDLance said, the only way to fix the OH issue is with an auxilliary radiator. Unfortunately something has happened with TX Chris and the V2 is no longer available.
The good news is that there is a set of plans and a detailed DIY is available here: Diesel Place : Chevrolet and GMC Diesel Truck Forums
I have one of the OH'ing trucks. Got hot pulling a 5100 lb travel trailer on SLIGHT rises in Nebraska when we first got it. Now a bumper pull TT creates more of an airbrake effect than a 5th wheel does because of the way the air comes off the back of the truck and hits the front of the trailer.
GM retro'd the 06 airbox assy in and I did the turbo mouthpiece and that fixed the fan running issue in hot weather as long as the speed is above 61 mph. Any less, and the temp is 95 or so and the engine heats up. Due to circumstances I was unable to take the truck into the mountains, create a video and beat GM in court like TX Chris did before he designed the V2. We now have a heavier trailer so I will be fabricating a setup using engineer837's DIY.
There is an issue where some of these trucks run hotter than others and some don't get hot at all where others will overheat and their owners cloud the fact by saying the OH issue is not true. There is something inherently wrong with the 04.5 and 05 trucks that hasn't truly been nailed down, but lots of contributing factors have surfaced.
1. the turbo mouthpiece is an abomination. Garrett redesigned it for GM for the 06 trucks and the part actually says Garrett on it. this is worth changing as I, and others have picked up mpg's by changing it and the airbox assy. I picked up 2 to 2.5 mpg by changing this and the hotside tube mentioned in nbr 4
2. the airbox assy sucks air from under the hood and when it gets hot from work, is creating a thermal feedback loop
3. the radiator is too small for the changes done to the truck for emissions purposes, GM increased the radiator 33% for the 06 model trucks and increased the fan fron 18" to 23" in dia
4. GM decreased the size of the hot side intercooler tube to try and overcome turbo lag due to the new for the LLY VVT turbocharger
5. it has been speculated that the trucks are too aerodynamic and don't shove enough air through the cooling stack. I have experienced this with simply increasing speed 4 mph and the resulting drop in engine temp and the fan not running. if all notice, GM increased the frontal area of the hoods.
6. as mentioned above, the tranny cooler might be too small here is a DIY for a tranny cooler, available from Summit Racing Diesel Place : Chevrolet and GMC Diesel Truck Forums
All the above plus a new freer flowing intercooler will be going in our truck this summer. We are also going to remove the cat in such a way that it can go back on due to living in an EPA inspection county here in Ohio. Every other yr our vehicles get inspected.
7. There is a possibility (speculation and thoughts on some folks that know more than I) that the turbo charger might actually be too small and is creating some of the heat issues itself. Garrett makes a drop in one that is bigger, but it is the priciest of the modifications. In a couple years I am getting one for my truck.