This has nothing to do with your starter problem but unless you have changed the balancing on your motor, a 427 doesn't use externally balanced flexplates or harmonic balancers. The 454 does. So if you used these items off the 454 and put them on the 427 you might want to get the proper items. Might save you some headaches down the road.
As far as the starter, are you using a shim all the way across or have you tried shimming just one side or the other? I'm with Tom. I have seem them take a bunch to come out right.
George