You have a Mercruiser correct? I believe that Barr sells a kit to do that, I would choose them over GLM. I did a similar conversion but it was on an OMC rigged 4.3. On mine there is a GLM kit but I did mine with Barr manifolds and elbows and Volvo Penta exhaust pipes and hoses. Cost more than GLM but better quality and longer life likely. I got my manifolds and elbows from Lighthouse Marine in Riverhead LI.
PS I used mine in salt water, had 3 sets of OMC batwings on it, over 15+ years of use, never had a failure or water in the engine but have heard of failures on both Merc and OMC once piece manifolds on these engines. I converted because the batwings have been NLA, for more than 10 years. I bought 2 sets back in 2004, thinking they would be NLA soon and was correct. In the salt we have to change at least elbows every 5 years, that's why I had gone through 3 sets, to avoid engine damage from salt water leaking into a cyl.
Yours might still be good given fresh water use, or not, I would take em off and look inside the outlets, to see how much flaking rust you have there, if they look really clean you may have a good set, but see what the Merc experts here say.
