I see Norm linked up my conversion. I made a chimney that extended the one on the new Dometic. The new reefers rely on all that space and the big louver type vents. On our trailers, Brian, we had neither. The louvered vents on yours were added by a previous owner I think. Our trailers used a solid door at the bottom that allowed access to the back of the unit and the burner. The heat from the coils was returned to the living space via the rear slit in the counter top. This uses a very simple system. In 1963
the heat and exhaust from the chimney was combined and sent out of that big duct that someone made reference to. With a little aluminum, a small brake, and a hand brake, you should be able to make a chimney to exhaust gasses out the original hole. Make sure the chimney extension has air space around it when it goes over the factory one. You want the exhaust to flow freely up not to be forced up. You can always use the big (sorry, terribly ugly) wall chase and long rectangular roof vent. You will easily find the cap, but the curb that vents through the skin will need to be salvaged or fabricated. It is is not easily seen, but this is a complex thing to fabricate.
Hope this helps you Brian. It all makes sense to me, but it might be a bunch to follow. Feel free to phone me, I think you have my number.