I've got 245/45r18's on my fronts and 275/40r20's to go on the rear. The 275/40r20 is almost exactly the same width as a 20x10" rim. It may be a little more sidewall than your wanting to run though since you're bagging it. I'd bet a 245/40r18 and a 275/35r20 would be a good matching set for a bagged truck. Then again, you could probably get away with a much wider tire in the rear.
Here's mine, rear tire unmounted next to the front wheel.