starter problems

[jodydewey]

I have a 1974 Chevy 350 with a TH350 installed in my 1968 shortbed fleetside. I keep chewing up starters and breaking the nosecone. I have tried shimming a brand new starter in 20 thousandths increments to 120 thousandths with no luck. I thought the flexplate was rounded too much so I replaced it with a brand new flexplate (FRA-100 autozone) listed for a 1974 Chevy C10 4x2 with a 350 2bl (VIN code Y). When the starter turns over it engages but after about 3 turns it starts to spin and kinda move away from the flexplate. I have tried 5 different part number starters and none of them seem to work. I checked the suffix number on my block and my casting number is 3970010 and my suffix code is CKH which means it should be a 1973 or 1974 Chevy 350 in either a truck or a Chevelle. Am I missing something here? I would have thought that replacing the starter and the flexplate should have solved my problem. HELP!!!!!