Hi meto i have done the same with shims on various truck over the years and found that starter gears vary in diameter (not much) and shimming works but you do have to re tighten the bolts.
I have also cut the length of the starter gear shaft so the gear comes further back into the starter. Also on some starters the mounting holes are elongated so you can alter the meshing of the gears.
If it dose not sound right when turning there is a problem.