Several Threads here on this topic, with a lot of good info. I had similar issues that developed going uphill or going slow, these were mostly tied to the weight of the Van (2006 Sportsmobile with the 6.0 // 11,800lbs & fully bulletproofed), and have been able to 'control' my temps doing the following:
**Mishimoto aluminum radiator
**Mishimoto Low Temp thermostat
**5-Star Tune (have a 40hp, 50hp, and 65hp - and now only use the 40hp) >>> Each Tune also engages the fan earlier, and this alone really helps on the long grades as well as with the slow uphill slalom roads.
Since I knew the fan would be running more, I replaced the belt tensioner, and the entire fan unit - already had a 2yr old BPD water pump.
I've been on the fence with the trans cooler - deciding if I want a larger cooler sitting in front of the radiator, or a remote fan operated unit located under the chassis somewhere (But will have one done sometime in late August). Currently the good news is when the engine fan activates it also pulls down the trans temp.
These are my Trans Cooler considerations:
ATS TRANS COOLER (WITH FAN)
https://www.atsdiesel.com/ats-auxili...3-8-inch-lines
DieselSite Transmission Cooler - 1999-2007 Ford Powerstroke 7.3L & 6.0L
https://www.thoroughbreddiesel.com/c...yABEgJcg_D_BwE
MISHIMOTO TRANS COOLER 37 ROW
https://www.mishimoto.com/ford-power...ler-03-07.html
TRUE COOL 40,000
https://www.oregonperformancetransmi...LNG-47391.html
My temps currently going up any sort of long steep grades don't really exceed these (because the fan kicks on sooner):
OET = 225 (before I'd reach 235)
ECT = 220 (before I'd reach 230)
Trans = 210 (Before I'd reach 215-220)