2003 Ford 7.3 non-dually automatic driver here. My auto lasted about 40k miles towing our 5er before it started slipping between 2nd and 3rd gear. This was with a larger cooler installed. I do have a chip which increases power and I don't baby my truck. I bit the bullet and went with a John Wood transmission and am not looking back.
My opinion and experience tells me that if you're going to tow heavy with a 4r100, plan to have to rebuild it at some point. And when it needs rebuilding, you need to spend the money on either a John Wood or BTS trans, any other rebuild is going to leave you stranded in short order.
The 6-speeds in these trucks are stout if you can find one and want to drive it. Keep in mind that the clutch on these trucks is much more taxing than on your smaller vehicles.
