Your maths is right as you only get half the lift as you are changing one end only. I would never run shakles that long as the extra leverage will rip the bracket clean off as some here have had happen. Take a look at the thickness of the metal! You will get lots of sway and rear steer from them as well even on the highway. You have also altered the roll centre a lot so your suspension pivots now at a different hieght to the front of the spring as well as the front of the car as well!
Hey country along with what Gojeep said if you don't have goos springs you may see more axle wrap. It will help with pinion angle when used with a SYE so it is a step in the right direction but using the full 2.25 in of lift out of them is kida iffy, your better off with good springs. IM me, I may have a set for you along with some other stuff you were lookin for.
i hate to bust in on this thread.....
how much lift can you safely get with shackles.......also anybody used the z-71 shackles........my dad-n-law is changing his to get the truck to sit level so maybe i could use his to get an extra inch or so........thats what i heard...any comments will be appreciated