Catalytic converters run very hot normally (remember the "do not park on tall grass" warnings), so I would not imagine a propane torch would harm it. I'd be more worried about adjacent undercoating, hangers, etc., catching fire. Be judicious if you go that route.

A word of encouragement. My darling wife ran over some plastic mesh/wrap in her Toyota, when it was new, and it fused to the converter. I couldn't remove it. I scraped off what I could and two weeks later it had "cooked" off, leaving only a brownish stain on the converter shell. It didn't smell too great for a week or two, but it returned to like new in just a short time.

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)