with 9.75 +20, I wouldn't look at any offset below 40. With that wheel setup, you're going to have massive poke, and unless you invest more in your suspension, you won't be able to get the excessive camber required to tuck under the fender. Plus the tires would be really stretched. I have 245 on a 9" rim and that's borderline for me, I would do at least 265 on a wheel as wide as yours. But if you're going for the stance look, and these are the wheels you want, you're going to at least need the suspension updates.
try putting some options into this calculator from the sticky listed at the top of this forum:
http://www.frsproject.com/