Both posts above are very accurate and you should check them closely. I did that and when I put my 351 stroker in the stock starter wouldn't turn it over cold. I don't know what compression you are runing but thye ground and timing are the best place to start. After doing all that and still having trouble I went to a starter place here and had them check it out. They built me a gear reduction drive starter and I have had it on since 1994 and have never had a problem since. Hot or cold it spins over easily on the first try. Since the car normally starts on the first try I very seldom have to try it more than once but I have ran it until it was good and hot and then shut it off for about 45 seconds to let the temperature peak and tried it again and it spun right over and started.
Ron61
Ron Widener