You have the ideal situation there. You have a set that fits, just see how much room you have to the springs/tub, and adjust the new backspace off the wheels you have now. So, if the wheels you have now have 5" bs, and the tire is close enough to the spring, just add the two inches to the outside, and keep the five inch bs. All measurements can be made from the wheels/tires you have now.
(Remember! Aluminum wheel width is measured from bead to bead, while ackspace is measured from the rear edge. So a 12" wheel is really 13 inches, and 6.5" backspace will be zero offset. The wheel will mount exactly in the middle)
