The catalytic converters I've worked on had a ceramic monolith with honeycomb cross section surrounded by a ceramic felt blanket.

I agree with P of D. Pinning the monolith with screws would only fix it temporarily. Fretting from vibration during use would destroy this setup in short order.

If John goes the 'destruction' route, he may be able to access the monolith from the exit side of the converter with a crow bar and avoid the cutting and welding.

Another option is to do nothing. As long as the monolith is intact, it'll still function. But once the monolith starts breaking up, he'll get exhaust restriction which will reduce high end performance (accelerating and high speed driving). Then he'll have to fix it.

Both (all four?) converters on my buddy's 98 ML320 are rattling. It's no wonder MB is at the bottom of Consumer Reports reliability list. Just another nail in the coffin.

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95 E320 Cabriolet, 169K