[BunnyPlanet169]

Looking at your pictures, I can see how the two pieces fit together, but WHAT is that gate valve for???

I think the benefits are as you've stated, with the trade off that it's big, heavy, and unwieldy. But, essentially no back pressure, and very difficult to get water back into the manifold - avoiding the 'keep the raw water intake off until start' directive.

I personally would not 'pressure' test. 20 PSI (most city water is at least 40 PSI) is a lot of pressure on an old copper manifold, and you'd just as likely blow out a joint as test anything.

I would leak test - put water into the water inlet, with a minor restriction at the outlet, and check to see that the exhaust part stays dry and water doesn't exit the jacket anywhere else. With the older cast iron sections on the output, visually check for corrosion. You have a late re-injection of water into the exhaust, and I'd always be a little worried about corrosion in that longer, still warm, cast iron (or is it still copper?) pipe section.

If you don't have one yet, get a CO monitor before you re-install into the boat.