This link should show you how to measure the clutch shims.
This elusive expensive tool (shim template) is 0.455" difference in where it sits on the clutch hub and where the feeler gauge is used. So with some care, straight edge, and depth gauge, you can do the math without this $175 unobtanium tool.
http://www.miraclemustang.com/photog.../V2D749056.pdf