Your tire manufacturer will list rim widths for your specific tire/size.
BFG, for instance, suggests rims from 8.5" to 11" for their mud 35x12.50-15, and lists the tread width as 12.5" on a 10" rim.
Based on that I'd say an 8" wide rim is right out and a 10" is ideal.
Backspacing and offset are two different ways of measuring the same thing, both based on the fact that the wheel mounting surface won't be at the center of the wheel.
Offset, IIRC, measures the distance from the WMS (the flat part inside the the wheel where the lug nuts bolt) to the center of the wheel, and backspace measures it from the inside edge of the rim, something like that.
Now, as for specific values, you'll find a range of opinions chiming in. Ideally as you go to a wider rim (i.e. 10" instead of the stock 8") you'd keep the same *offset* (but add 1" of backspacing, since the wheel is 2" wider), so that your scrub radius doesn't change. That is, the center line of the tire stays at the center line of the wheel, so that you're not trying to push more of the tire when you turn the steering wheel.
However, you can only move so far in before the wheel starts contacting brakes and steering components, so you can't go much farther in.
In practice, I don't know that a half inch or inch makes a hill of beans of difference to the steering, especially since you've got more (and prolly higher traction) rubber than stock anyway.
And don't get us started on tire fitment. 35"s could fit on your truck without ever rubbing, depending on the wheel backspacing/offset, spring rate, steering geometry, actual tire size, driving style, and phase of the moon. Or you might have to remove 2" of the wheel well with a Sawzall.
Even your tire dealer won't even try to guess what'll fit -- you gotta try it and see.
-- A
