the flexplate keeps grinding against the starter. the flexplate and 350 engine are pre 1980 year and the transmission is a 1989 700r4. Are these two not compatible or something? I've tried every single thing to getting the starter on right and it just grinds no matter what. The flexplate's teeth have wear only on the left (towards engine) side.

 

Would shims for the starter help? I got shims with my starter but never had to use them, but that was with a 91'LO3. Definitely not pre 80's......

 

Those should be compatible. I think bitchin_buick is on it with the shims. Sometimes you need them sometimes you don't. Try the shims, using the thinnest first and working your way up in thickness. If you don't have any, you can pick them up at most any auto parts store. Also check to make sure you are getting full voltage at your starter, low voltage can cause issues too.