255/30/20 will be PERFECT!!! Trust me I've done it. With that offset you are going to need all the help you can get. Your RIMS with those offsets will stick out about 11mm (approx 1/4") from the fender. Running a 265 will rub. I ran a 9.5(+21) with a 265 tire and -3 degrees of camber. With that my RIM was 2mm behind the fender and the tire still rubbed. And it is with camber, with a pull and a roll. The person that put my wheels and tires on didn't even have to use a cheater tank to air up my tire. Just a regular hose. And I did burn outs and all with mine and was perfectly fine.
This is a 255/30/20 on a 10.5 wheels