Most performance starters will come with several flat shims. Each shim is usually about 0.015-inch thick, and it may take several stacked together to keep the starter quiet. When installing these shims, you can loosen the bolt closest to the oil pan, but leave it in place and only remove the outer bolt.
Header heat can kill starters. Look closely at this upside-down picture, and you'll notice that the starter has been rotated all the way down so it hugs the oil pan for header clearance. Not all mini-starters can be clocked in multiple positions like this one, so be careful what you buy.
Sometimes the engine stops with its ring gear misaligned in relation to the starter's pinion gear. IMI incorporates what they call a "soft engagement" design and its pinion gears which are spring-loaded so they can slide back to allow the gear to turn slightly and engage the flexplate.
The flexplate can be the cause of many starter woes. This high-tech flexplate from Reactor Products features a replaceable ring gear and aluminum CNC-machined center. It's important to measure wobble on any new flexplate as shown. More than 0.050 inch is bad.
It's also important to measure runout on new and used flexplates. To do this properly, first mark off your ring gear in several equal parts. Then measure the gear teeth at each mark. Used ring gears may show more erratic readings then new ones, so it's best to check and log your results each time the engine is out of the car.