What you need to do is measure your current dual spacing. The easiest way to do this is simply measure the distance between the center of your tires.
The 255/80 has a listed minimum dual spacing of 11.2". The minimum spacing on a 255/70 is listed as 11.6". The 275/70s being suggested have a minimum spacing of 11.9".
I do agree the 255/70s are too short. Your engine will be turning too many RPMs. The 265s recommended in other posts fit well but they don't give you any increased carrying capacity.
Do you have any clearance for a taller tire? A 10R-22.5 is 1.9" taller but gives you 470# more carrying capacity and has a 11.1" minimum dual spacing.