@op, you want to keep the same rolling diameter. So instead of 255/35 F, and 285/30 R, go with 255/40 and 285/35. When you multiply the bigger number by the smaller number, which is percentage of bigger numbers tread, they should come out close to 1:1 ration.
255 X .35 = 89.25 fronts. 285 X .30 = 85.5 Rears; whereas; 255 X .40= 102 fronts & 285 X .35= 99.75 rears