I agree with fordf350 if possible you should run thermostarts. Have you tried the boat with thermostarts?
Keep in mind the new motors from Mercury are running hotter and now a lot of them have a closed cooling system ?
Let us know you end results.
Thanks Brian

Those aluminum gaskets you're referring to sound like block-off plates. People use those sometimes when they can't keep the joint between the manifold and the riser from leaking back down into the manifold, so they install a block-off plate and use the freeze plug holes to install hose barbs and a bypass hose to go around the joint from the manifold to the riser. I would make sure you haven't started leaking water by taking that out.