You could just pressurize the cooling water side with air, and a pressure gauge instead of making plates and block off's.

Yep. And a lot easier to spot tiny bubbles than a few drops of water on a header that is already wet. I plugged all extra holes in the water jacket with small pieces of hose plugged with bar steel stock. I rigged a fitting to one inlet of the water jacket. Put regulated air pressure in and wait awhile for any small bubbles. Works great but you do need an air compressor.

That method works great if you have water dumps on the collectors or manifolds. Mine had four openings around the circumference of the collector. No easy way to block those off.