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I agree that the furnace shouldn't be trusted, and I seriously doubt that it was OEM from Airstream. Most furnaces utilized in Airstreams the size of an Overalander, particularly by the 1960s, were totally hidden within the cabinets with ducting to at least three or four places including the living area/kitchenette, bedroom, bathroom, and holding tank area. I have seen a furnace resembling that one in a friend's canned ham, and it wasn't ducted so I wonder what may have happened to the OEM ducting system -- that may be something that could complicate the installation of a new furnace -- also be prepared for the possibility that the exterior skin may need a patch as most of the new furnaces require a smaller intake/exhaust port than the OEM furnaces (the size difference was remarkable on my '64 Overlander).
Good luck with your investigation and project!
Kevin D. Allen
WBCCI (Lifetime Member)/VAC/Free Wheelers #6359
1964 Overlander International/1999 GMC K2500 Suburban (7400 VORTEC/4.11 Differentials)
1978 Argosy Minuet 6.0 Metre/1975 Cadillac Eldorado Convertible (8.2 Liter V8/2.70 Final Drive)