The starter drive gear will be too close to the flywheel, causing a grinding noise when the starter is engaged.
If it has shims now and you buy an OEM replacement starter, then use the shims. If it does not have shims then you will not need them. Replace the starter with an exactly duplicate, installed exactly as the original. If in doubt measure the distance from the mounting plate face to the end of the drive gear shaft and make sure that distance in the same on the replacement starter.
Need to know why you think the starter needs to be shimed.
Shims are used to fit the upper control arms, the front fenders, the starter and so on. The number of shims varies for each application.
Do not install the shims. Your starter should work fine.
between the starter and what it is bolted to
They sell shims because there are differences in the castings, so there is no right answer to your question. Most of them do not need a shim. I have never needed more than two.
You should be able to buy starter shims at any auto parts outlet. If not their you could get them at the dealers parts department.
Check the backlash between the starter drive and the ring gear after installation. If it's too tight, use shims.
One of the easiest starters ever... 1) Disconnect negative battery cable 2) Remove nut holding power wire to starter. 3)Remove smaller wire(s) to starter. Mark which goes where if several are present, and you don't think you'll remember how it goes. 4)Remove the starter bolts that are locatd directly under the starter. 5)Starter will come out of vehicle at this point. NOTE: Many Chevy's use shims to space the starter from the flywheel. If your vehicle uses them the new starter will likely come with shims. Use the same number of shims as was in the vehicle when you took it apart. If starter grinds when starting after starter replacement you may need to add additional shims.
Disconnect battery. Then the electrical connection on the starter solenoid. Remove the starter bolts and the starter. Note the location of the spacer shims. Installation is the reverse. Don't forget the spacers shims or you will get noise when starting the vehicle.
There are several shims that could be used such as starter shims (for adjusting drive gear to ring gear), caster/camber shims (for front end alignment), body shims (for aligning body panels),
Shims bring the starter closer or further from the ring gear for proper alignment.