Once the plenum was in place, the side "take-offs" could be rivited in place. Note the space around the sheet-metal. See the hole cut in the upper left of the box for convection cooling. Mounting brackets are added and the adaptors a screwed into place.
I will find later, that I really need to install another duct to move more air through the plenum. That made heating more efficient. The heat limit switch would kick in and shut down the combustion from time to time.
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