265 tires on GM stock wheels

I went to COSTCO to buy new tires. I wanted them to install 265/75/16 on my 2005 Sierra. They said that they were only allowed to install 245's on the truck. I know that many of you (and myself) currently tow with 265s.....have any of you had any trouble using your larger tires on the narrow (6.5") GM wheels? Do you really think the 400lb carrying capacity of the 265s is a reality with the 265s? Why did you choose 265s over 245s?