The Dupree manifolds are way too thin to drill into. I drilled very carefully into the thick, flat part of the exhaust collector of the turbo, and then put grease on the bit after it was about to break through to pull the shavings out cleanly. I put a shop vac to the exhaust part of the turbo to get anything else that may have fell in there. For reference, these were JWT 530 turbos, brand new. I ended up taking the turbos off the engine and there was absolutely no metal in them with this method. Good luck, all the other posts show good places too, just depends on if you have the stock manifolds or not.
