I have a 2001 4.3 mercruiser/alpha, when I separated the outdrive to change the impeller I found a few chunks that looked like cast iron from the exhaust. Turns out I have the one piece style manifolds. I pulled them off to change the exhaust shutters but didn’t see any chunks missing. Is the manifold most likely where it came from? If it were your boat, would you spend the $1000 on the conversion kit? This boat has been a massive headache and still have not had it on the water after buying it “water ready” in August. Learned my lesson