275/70-18 offers an inch taller then stock 275/65-18's. That size will give you many different tires to select from and they tend to be cheaper because it's a stock size for Super Duties with 18's but they are only available as E range LT tires.
I ran this size on my 07 and have been very pleased with the look and all.
if you want big tires just put a level on it and throw some 35's under it. i got my 2.5 steel level kit for $60 shipped from hellbentsteel.com. another 150 for install and alignment. wheels and tires m&b w/ shipping fro 1300 so total in was $1510 to go from stock to this