Backspacing is the distance from the back of the rim to the hub mounting surface, and the stock dimensions is 3.75" for 7.5" rims and 4" for the 8" version.

Offset is the distance the hub surface is from the centerline of the rim. Positive offset moves the wheel in under the vehicle(hub nearer the outer rim surface like the new trucks) while negative offset moves it out(hub nearer the inner rim). Stock offset is 0(zero) with the stock wheels.

With these trucks you want to keep backspacing at 4" or less regardless of rim width, any more will increase tire contact on the radius arm.