On that copper sleeve technique..That must have worked its way south as I had a head porter that did the same thing but he did it azz backwards. He would port the bejeebers out of the pushrod guide side and break into the passage like it was nothing. He would then drop in a sleeve with epoxy and let it set up and smooth out. Your way is more proactive as it does give you a warning. Bring on the T/A valve gear!