Well, the "Sunbeam" gravity furnace (circa 1960's) in my 1880's victorian home was red-tagged by the gas company. I think the culprit is the pilot system. No one wants to try to fix it. Does anyone know a hvac contractor in Orange County, CA, who will work on a gravity furnace? And, where can parts be obtained?
I hesitate to say you are beating a dead horse, but, why are you beating a dead horse? Could be time for a replacement unit, and if you are concerned about those huge ducts, forced air ducts can be routed right through them.