Quite sure the spacer is only for the early engines. Yours should be new enough to not need it. You're probably seeing a 1/8" difference in flywheel position. I measured a stock small block with flywheel, and compared it to a 2010 ls with a 2005 6.0 flywheel. The ls engine spaces the flywheel surface 1/8" further forward compared to the old sbc engine. I'm using an adjustable pivot ball in my sm465 bellhousing to compensate for this 1/8" difference. It should work well, but is still not assembled(I got sidetracked on a AFM delete).
My bellhousing is an external hydraulic slave. If you have mechanical linkage, you might be able to compensate for that flywheel height difference in the linkage alone. You also might also reach the limits of the bellhousing window, by doing that.
I ran across a thread of some guy in Canada who installed a ls engine in an external hydraulic slave truck. He did not install an adjustable pivot ball, and reported the truck usable, but he had to either mash the pedal all the way to the floor, or extend the slave pushrod. I forget which.
Mr. Gasket 3885G Adjustable Clutch Fork Pivot Ball
Look for my post
here.