Some older GM cars may drop a couple of shims in your face when you remove the starter. Whereas most starter motor replacements can simply be bolted on right out of the box, GM engine blocks and starters are dimensionally inconsistent enough to require shimming for proper tooth engagement. Improper tooth engagement leads to a myriad maladies: the clearance will be too tight or the teeth may not engage fully, giving you gnashing teeth instead of the hummingbird whir of a cranking engine. The best place to start is to replace the shim or shims that came out with the old starter. Don’t even bother to hook up the wiring yet. Remove the sheet-metal cover over the bell housing so you can visualise the starter’s teeth and the ring gear on the flywheel or ex plate. With a screwdriver, pry the bendix gear forward to engage it with the ring gear. While keeping the gears meshed, measure the gap between the teeth with an 80-thousandths wire gauge. Surprise – there might have been such a gauge included in the new starter’s box. If not, a paper clip is pretty close. If the gap is less than 80-thou, add a shim. If it’s more, remove one. You don’t need to remove the starter motor to do this – that’s why the shims have a slotted end. Loosen the starter bolt closest to the centre of the engine, but remove the other one. The inner bolt will hold the motor up. Now you can slide the new shim in (or the old one out). Reinstall the outer bolt, tighten the inner bolt, and you can recheck the gear engagement.
Occasionally, a starter motor will have proper radial clearance, but will still exhibit incorrect engagement. It also may be necessary to shim the bendix gear to get full engagement. I’ll refer you to the shop manual.