Youre going to need more than the dry vent to get the cold air back
I copied an existing setup to some extent, a factory unit where the cold was blown in thru a 6 inch and leaves thru a 16x16 plenum. JT, I realize you can build a non leaking unit, the same way this one is but the layout made this easy and like you said, corrode thru is the thing or possibly a heat stress crack. The factory unit of course had grates on sides, etc but basically a sheetmetal firebox where it would be more concern.