I know the black marker didn't show much, but I'd still open up that slot width. The black marker (similar to dykem) is a machining tactic for knowing when you're making contact between cutter and work piece. If there's not enough abrasive action, it may not remove the black. That was just something to try, because if it did show wear, it would be obvious.
In my opinion, altering a $13 part is cheap insurance to make that body (I'm assuming $100+) work.