A good starter ought to be able to crank a cold 350. Do you have the timing advanced too much? What about charge level of the battery? If the timing isn't correct or the battery is too low to crank, you'll have problems. Not the best test, but a quick test you can make to check to see if timing is a problem is to disable the coil and crank the engine, of course with the spark plugs in place.

Also, are the terminals clean, meaning shining like chrome, not just clean of green jelly. I mention this because I have ran into so many issues where these things are the problem, and the person didn't ensure that these things are correct

On to the next step: Is the starter an ACDelco rebuild? Other-than-ACD rebuilds have their own formula as to where the armature is undercut. That spec can be any size they choose. ACD rebuilds have to meet GM's specs for minimum armature undercut. Advanced sells delco. They might not stock the parts, but they can get a lot of delco parts, and they could probally get a delco starter if you requested one.