If you have to use starter shims, either there is an obstruction (dirt, grease, debris) on the mounting surface of the starter or engine block or the drive end housing of the starter is worn so badly that it needs to be replaced. Additionally, the correct starter bolts are specifically designed to aid in proper alignment of the starter drive to flywheel or flexplate for proper tooth engagement. One last item, depending on your application, there is a bracket that bolts to the starter housing or end-frame to the engine block. It is also designed to aid in proper starter-drive alignment by supporting the weight of the starter. I've been working on GM's for 50+ years and have never shimmed a starter, but have replaced countless cracked and broken drive-end housings, correct starter bolts and support brackets. Seriously, if you make 100% sure that both the mounting surface of both the starter and engine are 100 percent clean, are using new, proper starter bolts and the correct bracket (id applicable,) you'll solve the problem as most of the drive eng housings I've replaced were cracked or broken from overtightening the incorrect starter bolts against a dirty surface. Hope that helps...
