I recommend a turbo if you go with the 4l80e on a 6.2. I did the 4l80e swap on my 1009 and without a turbo it could not stay in 4th when going over an overpass in Houston. Not enough HP to run the tranny in 4th other than flat land and down hills which it was great for that. I changed to a 6.5 turbo and it is a night and day difference with the shifting and drivability of the truck.
Others may not have this problem, My 6.2 had new IP, Injectors, compression test were excellent so the motor was not old and tired and I am in Houston Texas at sea level. At altitude it would have been a real dog.
Not trying to change your mind in any way just giving my experience.