Years ago I had the brick apart and ran into the same observation. Used a large piece of glass pane on the bench with White Diamond polish to "clean and true" the surfaces. Easy to do with even pressure and pre-clean with acetone and a plastic scraper.
Enjoy, Cheers
* opps, noticed it's the block side .. I'd still use a small flat glass and compound, but very careful not to spread a mess into the engine.