Another vote for the 6.0 trans cooler, you may want to consider a transmission temp gauge as well. We added a Bully dog tune to ours (set at the lowest hp towing setting) and then learned the hard way that just because it will scale a mountain in overdrive at 70 with a 31' travel trailer doesn't mean it should. Two transmissions later, we've slowed down.
We also added firestone rear air bags, helwig anti-sway bars, bilstein shocks, and double husky sway dampeners. All were a definite improvement.
I've read that if you replace the stock springs with F250 springs it will really firm up and tow nicely. The DW did not want to firm up the ride, so we did not do that upgrade.