I would agree with A & C here.All the CSU's I have had any thing to do with are fitted to the engine.I have never worked on an Aztec mind you.Have another word with the prop shop doing the notice 75.As for a three bladed prop your best bett is to e mail the likes of McCauley and Hartzell and ask them if they have a three bladed prop with an STC for your aircraft.
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