BEARINGS?

Merely replacing the main bearings isn't usually successful. Have you had the crank journals inspected to see if they need cleaning up ... out of round, tapered, etc.? That should be done, then once you know what the new journal sizes are going to be you can have your original bearings rebabbited. Line boring of the newly babbited bearings isn't quite as critical with the sixes as it is for the eights, but still the best procedure. Plus it may be hard to find anyone who can machine the rebabbited ones to correct size without their being installed in the block.