Those manifolds were poorly cast & prone to failure which can ruin the engine, it’s risky to keep using them. The OMC version I had didn’t seem prone to that problem & I had that set up on my engine till 2016. The Barr kit for the Mercruiser works with the original Y pipe. To replace the hoses you must remove the manifolds first. Lube up the Y pipe with silicone spray & slide the new hoses down to the top of the Y pipe. Reinstall the exhaust manifolds & then pull the hoses up to connect with the manifolds. That’s how I did it on my OMC. If those hoses are burned then the manifolds are clogged with rust & MUST be replaced.