After making sure there are no irregularities on its surface, I would put spotting blue on the breechblock shoulder and keep trying it in the receiver to see where it is touching.  Use a strong light and riffler files to take off the blue spots until the block rises to full height and contact is complete.

Tedious, finicky work it is, but I don't see an alternative, and the result will be a good fit with full contact.  Check the fit of the tail of the breechblock in the tang as well to make sure there isn't any casting flash keeping it from sliding back and forth smoothly.