Need starter help bad

[BowTieMan]

Ok I have a 70 with a 350 and auto. When I bought it the owner told me that the starter on there was special. And if I bought a new one I would need to take the nose end and the gear off and jsut get the new motor. After about 2 weeks of owning it my starter wouldn't work. So I replaced the whole thing with one from autozone. I didn't shim it and it wokred ok at first but wined alittle. Three months later out of no were it starts grinding and screaming when I try to start it. And made that horibble sound when it hit the flywheel. And my friend said sparks came out of the bottom of the truck. So I put a shim in and it still wined alot. So I got a new one tuesday night and put it in. And I added three shims and it wokred great, was quiet and started right up. Took it to school today and it was perfect, drove around to work and walgreens. then I went to go out tonight and it did that same wining sounds. Is it possible that the starter is geared for the 153 tooth and I have a 168 tooth flywheel and it is jsut to hard for the starter to move it over at first? I am going to check tomorrow what flywheel I have. Also we checked the flywheel and it is fine. I am at a loss. First one was three months and this new one is one day. Please help me. Any Ideas?