I haven't seen you on for a while; hope everything is O K.
I agree with Guyod, your furnace has been faithful to you and paid its way. It is now at least 30 years past its economic life and there is no need to keep investing in it. A newer, high efficiency furnace can pay for itself in 5 years or less. Its time to call in a couple of favors and get a new one. Don't forget to search eBay for one, you never know.