call me crazy !?

Typically the studs will only accept about a 1/4 spacer.If you need more, you will need to punch out the studs, fabricate metal blocks and fit longer bolts.
Alternatively you can move the axle back on the perches, by elongating the locating holes in the perches. keep your eye on the rear shackle angle. I had to move mine back, as the springs were de-arched and rather flat and long.
Also, as mentioned, keep an eye out for the shocks getting into the gastank.
And yes, it centers the axle in the wheelhouse to make room for 28plus inchers.