Def. on the right track!
Although I haven't replaced or modded my enginebay as you have, I have been messing with the 4L60Es cooling, gauges, etc for over a year now.
A few ideas that you may or may not have considered. One add the deep pan trans and filter kit (the same that is/was found on the 2000 Silverado). If you need the part numbers for the pan and the deep pan filter kit, let me know. The GM pan does come with a plug, though it is darn near impossible to remove.
The reason I mention this is because it made sense to me to have a bit more volume of tranny fluid on call to help curtail some of the possible flash heat. It seems to have worked as I recently towed the Safari (fully loaded--6300lbs approx) up a steep one mile grade. The trans barely got to 200 degrees only after I got to the top.
Also there is a torque converter shroud that replaced the metal one that I had. It should fit on any 4L60E. What it does is simple, it has a "ram air" scoop that captures air coming from the front and circulates it around the torque converter and then out the side. It's made by Neal. It's not expensive by any means and you can see a pic of it here:
Click on the Neal Torque Converter Cover link on the page above...costs $59.
I can't say that these two are the sole reason my trans temps stick around 175 degrees when towing our 25' Safari, but I am happy to have them. I also recently added Water Wetter to my cooling system (one bottle). I swear between that and the LT1 160 degree thermostat, the engine while towing, even in 85 degree heat, stays plenty cool! Of course if your truck does not have reverse cooling, I might not install a 160 degree thermostat.