What's the difference between a a/t starter and m/t starter?

Just like the title says. I have a 351W and toploader in my car. I've bought a couple of starters for it at Autozone and have always had to buy the automatic trans starter. The manual starter would not fit in the hole in the dust shield. The lip on the starter was too big in diameter to fit in the hole. The automatic one seemed to be a little smaller in diameter and worked fine other than hot starts.

I'm looking to upgrade to a high-torque starter off ebay to cure the hot start issue. The company selling them told me the difference is in the length of the throw on the starter drive and that they will not interchange. Now I'm confused on what to buy. Anybody else run into this problem? I'm wondering if Autozone may have the part numbers reversed in their computer or something.
 
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