Nooo. Don't take it out of the radiator on an Allison. If anything just add a larger external cooler. Not only will it help trans temps, but it'll also help pull some heat out of the coolant. It helps warm it up (definitely important in a pickup so the converter will lock up, trans fluid temp has to hit 100 degrees before it locks) and also regulates it. It doesn't make much heat once the converter locks anyway.
Also, the factory gauge won't go above "210" (on a pickup) even when the coolant temps get in to the 220-225 area because it's still considered normal operating temp. They're designed to give you a false sense of security
My actual coolant temp is 186 when the dash needle it pointed straight up at 210 like normal. It's kind of a....suggestion
FWIW, with Hayden 2886 fan clutch, a larger PPE trans cooler, and a 2020 oil cooler, I've never seen coolant temps over 199 degrees as read by a CTS monitor through the ECM. Gross weight was around 23k and pulling uphill in WV. I know the MegaHauler is different, but you're definitely on the right track.
Running along empty, my coolant temps are 186-190. Stock trans temps were always 170-ish after the cooler. I'm sure this newly built one will be a little higher as it runs noticeably higher line pressure and will hold full line pressure even when the converter locks. Should still be 180-185, I believe. We shall see!