You need to measure the distance from your mounting plate to the front edge of your flywheel teeth. The long end if some dial calipers are great for this. It needs to be within 1/16" of spec, depending on your motor and starter model. If you're distance is under spec, you as the shim.
I'm a Family Practice doc in Ft. Worth.
Edit: Here are the instructions for your starter:
http://www.powermastermotorsports.co...ockingInst.pdf