If you have to replace the cat you might think about getting something like what I'm using.
https://www.magnaflow.com/products/091-1016-catalytic-converter-federal-universal
It would have to be welded in but smaller than stock. Of course this only works if you don't have to be emission tested.
Also I'd check around and see if you have a local shop that does ceramic coating as it really wasn't that expensive.
Your other option would be to pull your tub & spray the underneath with Lizard Skin.
Sorry I missed this. Thanks for the ideas.