Get rid of the spare tire (dead weight anyway), put in the cooler of your choice where the spare tire was on the brace. Run the cooler lines aout of the tranny through the frame on the pass. side. Assist the cooling at lower speeds with a puller fan.
I have a huge cooler in back, with a 10" puller on it. Trans temps with the 9/11 ride between 150-175 in 85-95 deg weather.