Been through this a few times and here was my fix.
First off, make sure you have the correct starter bolts.
No washers on the bolts and make sure the bolts are clean.
There are 2 milled flats on the starter where the bolts pass through, the shoulder of the bolt rides on these, make sure both are clean. Install the starter with no shims. Torque the bolts down to 35 ft lbs.
This should take care of it. Adding a factory starter brace would likely be a good idea as well. You can still get these from GM.