I use High temp anti-seize compound, Nickel compounds are best but more expensive than copper, which will do in your case unless it is turbocharged, then use Pure Nickel compound.
The catalyst support ( the ceramic you see) breaks up because of thermal shock. This was a big problem up to about 1980 ,then better support ceramics were developed that don't breakup. I emptied about three converters in vehicles older than 1980, just remove the large Allen nut on the bottom for a couple weeks. I could no longer get on the expressway because ...
Most cats get blocked with a combination of combustion products and burnt oil.
This happens due to two main causes:
a worn engine allowing oil into the exhaust (valve seals, rings etc)
the wrong oil being used - this is a common issue as people don't check for the correct grade of oil and, in many cases, opt for the cheapest.
Some cats can be cleaned but ...