Ok well I learned the hard way that I messed up with my spark plugs, but I'm also having starter troubles, I just burnt my 2nd starter in 3 months, I thought the first one went because I got it used from a disreputable autowrecker.
now my AC delco reman went as well. It's been shimmed properly but it sounds like its grinding on something and barely turns the engine. The flexplate however shows no sign of wear.
wtf!?

 

This is gonna sound a little nutty, but there are some GM blocks that weren't drilled properly. Try shimming the starter so that it leans towards the flexplate, just a little. I had to do that on my 79.

 

BTW, check clearances first, physically get under the car and move the starter drive out towards the flexplate, see for yourself if it's engaging correctly..