Am I dead in the water

[jaluhn83]

It would be a PITA to clean the mating surface of the block and not get debris down the hole with the gear sitting there.

Something to be cautious of, but not too hard. Clean thoughally before pulling the gear cover, then just use good work practice and clean away from the opening. Any small amount of debris that does drop in is very unlikely to cause any damage - it will drop down into the oil pan and unless you drop enough junk to plug the suction screen or super fine abrasive that passes through the oil filter it's not going to do anything from there. I'd be more worried about dropping tools/hardware down there myself since that could in theory jam the gear train (apt very unlikely)