If you pull the front O2 sensor and install a sensitive pressure gauge, a converter that is plugged will show positive pressure. Anything over 1 psi at idle is bad. Rev it up and you might see 5 psi or more. Dead giveaway the cat has done a melt down.
Since the rings are worn, the amount of oil run through that cat has likely melted it. If you find pressure, you can open the exhaust right before the cat and give it a quick run… in the day… it will wake up your neighbors.
If it is clogged there might be a little more life in the old girl.