The rubber crossover tube in the back should have a RED STRIPE on it. If it does, it has had the recall done on it, if not it is the original and there is danger associated with it, as the tube can crack and combustion gasses may leak. Suburban no longer supplies the recall tubes, or any other parts for the old NT series furnaces.
Check the combustion chamber for rust with an ice pick type tool, especially along the bottom. It may be quite hard to see it to check without careful disassembly. Press the tip of the tool against the metal and be sure it does not punch through. Any indication of that happening tells you that the combustion chamber is bad.
If it passes those tests, I would remove the cast iron burner from the back of the unit and run a hack saw blade through each of the slots in it to remove the rust which often clogs up the burner slots. Then reinstall. Most of the old cast iron burners clog up with rust after a while.
All in al you are dealing with a 40 year old furnace, the proper operation of which has your life at stake. Please be careful.
A final story: I had a '71 Caravel
with a difficult furnace. I finally got the parts and got the pilot light system fixed. It ran fine, made good heat. I replaced it in the trailer. Every time I used the furnace for a while I would get headaches and feel crappy. The windows also fogged up more than they should have. I put two and two together and figured the combustion chamber had a leak in it that I could not see. I replaced the furnace with a new one. No more headaches. Vastly reduced condensation on the windows. The gasses from burning the propane were the problem, water vapor is one of the products of combustion, and the leak was putting lots of additional water into the air in my trailer. And obviously, there was some CO getting inside too, causing my headaches.
Again, please be careful.