Hello NDARGOSY -- and welcome to the Forums! Madison -- with a daughter working at University Hospitals I get there with some regularity. Were photos of your Argosy at Capital Springs on the Forums a few months back? Somebody's was...
All gasket materials get hard and cracked over time. I saw a '58 Airstream last week with gaskets as hard as [insert exaggeration]. I don't know your specific answer because I haven't gotten to this point on my Argy. But I have to throw out a caution. More than gaskets may be at issue here. Search the forums on 'Suburban recall' -- for example, here
. Evidently back in the 70's they recognized that an air intake (or exhaust?) plenum was cracking prematurely. This could be a significant danger and I'd almost encourage a new furnace unless you are working in very familiar ground. Even the most vintage-friendly Airstream dealership (like Fogdall's in Iowa) would probably shy away from working on any of these old Suburbans!
Just looking at the returns on the recommended search I see a thread about pilot light going out too, but think you'd be more successful searching on that as a separate topic.