[Rich Biebel]

Stock GM starters were often a horror show to make work without the problems you have.

Been there a million times in my mechanic days

I tried everything including using a rat tail file in the starter mounting holes to move starter closer or farther away from the block.

Tried different starters with various noses....rebuilds usually. There were some different noses, some even were cast iron noses and manual trans might be different than an auto flexplate

There is no easy answer.

Even the aftermarket gear reduction starters can do it but shimming usually fixes it.