I had a 99' F250 with 6spd trans, the only things I had done to it were intake/exhaust (stock downpipe), and a 60hp tow setting flashed onto the factory computer.
I can honestly say it was the best towing vehicle I have ever owned or driven (of 3/4-1 ton trucks).
Those vehicles include:
2008 King ranch with 6.4 (rode MUCH better, but if you don't run 80 mph it seemed weak)
2003 3500 Dodge with automatic, intake, exhaust, and chip. Sounded HORRIBLE (5" straightpipe), transmission SUCKED, had good power but I wouldn't have one of those POS trucks to 'keep' (I traded another truck for it)
2005 F350 6.0/automatic, all stock, drives great and is fast, but with a trailer behind it I am not too impressed, better than a stock 6.4 maybe?
I currently own a 2003 6.0 Excursion, 4" turbo-back exhaust, tuner, on the 60hp tow setting I would much prefer the 7.3/6spd over the 6.0. The 6.0 is FAST (for what it is), but towing a heavy load the 7.3 does a much better job.
I towed a 36ft enclosed with a buggy, ATV, dirtbike, etc out to KOH and back in 2012 with my F-250, and I will say the truck didn't bat an eye. Wouldn't win any drag races while pulling, but it was a strong truck and I miss having it, just couldn't fit a carseat and a dog in the backseat.