18" & 20" Setup

I like the look of the cars using 18" in the front and 20" in the rear American Racing torq thrust wheels. My question is what offset is needed for this setup? I assume the fronts are 18x8 with a 0 offset and the rears are 20x8.5. But what offset for 20" +0, +18, +20, +32? Also what width tires will fit? 245 in front and 275 in rear? Thanks

for 1971 chevelle, lowered 2"