That's the way to do it if you want SOA. One thing to think about though, you're going to have to lower your bump stops. You will be moving the bump point (axle) a few inches lower than since the axle is now below the spring. So you've just increased the bump point by the axle width plus the old spring perch height. So unless the springs you are going to use are designed to go into a negative arch you'll have to move the bump stop down or add a bump point on top of the spring.
If you plan on using your old springs it's easy to measure to see how high it'll sit. You'll raise it the width of the axle plus the height of the old sprinng perch plus the height of the new spring perch plus the thickness of the spring pack. My guess would be between 5 to 7 inches in total.
SOA Rear
