Starter shimming frustration

What is the proper way to shim the starter motor before damage occurs? I just tore up my old starter gear because it wasnt shimmed out right and, for the new motor, I dont feel like ruining also. p.S the flex plate is new aside from the damage from the old starter crapping out. The engine is 10.2 compression so it is harder on that poor starter gear. I tried to run the new one and it makes horrible sounds and the gear is wedged so tight against the flexplate that it doesnt retract. Thanks guys

first of all do you have the right starter-flywheel combination,there is a 14" which has 168 teeth and a 12-3/4 with 153 teeth flywheel or flex plate,each takes a different starter nose housing. The 14"flywh. starter mounting bolts are offset,the 12-3/4 the bolts are across from each other. If the combo is right than just pickup some shims and try one at a time, see what happens,could use as many as maybe 1-3 shims, I believe I've haven't used more than two at any one time.