starter problems

i need help i have a starter problem ive went through four new fly wheels i tried many different ways on shimming the starter i even have a cast iron head with three bolt holes on it it worked for a while but started to grind again and the starter is starting to flex is this the problem. will this make it grind. do i need the brace that comes with some starters and will it really fix the flexing problem no matter what fly wheel i put on or starter it starts to grind till it wont start anymore just grind really bad i have it in a 65 chevy impala ss, 350 with a th400 can someone please help me im tired of unbolting this tranny.