I don't trust automatic transmissions, especially for towing. Mopar trans are really bad and often fail around 60k miles. If yours is working well with no signs of slippage or flaring I'd throw the biggest stacked plate cooler you can find on it and not sweat it. For your next truck get a manual trans. New Venture makes some really stout transmissions. Ford uses ZF which used to be really weak (like in my truck, only rated to 420 ft/lbs... engine makes 550 and it hasn't blown up yet with plenty of towing), but the newer ZF transmissions are rated for a lot more torque. I would never own a truck with a slushbox, it's just inviting trouble.