Oh yeah the torque capacity of the allison is supposedly 660ft/lbs, but will handle more than that, especially in a small truck as upposed to towing a big trailer.
I wish I had more experience with the 6l80, sorry.
i have a 4l80e and in order to use the transmission you need an aftermarket computer that is around 800 bucks. i have read several articles on it and its not recommeded for a small block unless your making serious amounts of power. it robs more power from the engine than a lesser transmission such as a 4l60e. it would be the same difference between a turbo 350 and 400 except they have overdrive and are electronically controlled. i have had mine now for about 6 yrs. and i bought it used then, but its behind a big block making over 600 hp in a 70 chevelle. if i can remeber the custom transmission shops name i will post it. hope this was helpful.