Well, assemble the rotor, extension shaft, and retainer. Snugg with wrench. Measure with feeler gauge how much actual play is there between the rotor and extension shaft. Add .010-.020 to this dimension , which is what needs to come off the splined end of the retainer. If it's the 08 model, chuck the big hex nut and machine the required material off. If it's the 08A, the sleeve needs to be pressed off of the splines. Now same proceedure on material removal on the end of the splines.
Recently went through this with a member x1rider which had an S&S crank not allowing the comp to snug up. In this case I blamed the crank as being a tad too long. What the dimensional error is in you case I don't know. Do not consider using shims since this alters the spring pack and preload dimensions. The basic problem here is possibly a slightly short spacer behind the rotor, one of the comp components are out of spec or the crank is a tad long or a combination of several. Thinning the spline length on the retainer will most likely solve the problem, once you see how much after measuring the free gap.