I have been a G.M. Guy forever. I have done the starter thing every way you can except sleep with it. I use a remote starter switch , Sun equipment, yes I have Some Age. cwc has a very good idea, you have to look at the engagement to see if they are square to each other. Yes I have single side shimmed before, bend a ninety and placed. Starter bracket shall be placed on EVERY starter, gas and especially diesel.
Unfortunately 6.2 has this problem of cracking the starter bolt casting area. You have the right direction, the starter bracket is also used to help align and square the starter.
If there is not enough clearance the starter will bind. The first thing I would do is take the starter drive, either a new in the box or off the starter and mesh it on the flywheel.
Look at the clearance and make sure they are compatible. Their are several different drives, the starter solenoid will have a definite effect. Even though it works does not mean that it is able to do the job. I just had one sticking and replaced the solenoid and spring, problem gone.
I have over the years collected 20 plus starters over the years and when fabricating for engine installs, use different starters for fitment. High torque, regular duty, gear reduction the list goes on.
Just because the part seller says it is the right one does not make it so. Original equipment part new or used is the way to go. I would make one before I would put a Chinese made part on my equipment.
I hope this helps, but to put more than two shims on there is a definite problem. Ask for a replacement starter, starter drive, or solenoid. It also matters how you torque it down, evenly or which one first. Gooood Luck!!!